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<^THE 


EMBRYOLOGY 


Blatta  Germanica  and  Doryphora 

Decemlineata. 

BY 

WM.  M.  WHEELER. 


Reprinted  from  The  Journal  of  Morphology,  Vol.  III.,  No.  2,  Sept.,  1SS9. 


BOSTON : 

GINN  AND  COMPANY. 


I 

i 


THE 


"embryology 


Blatta  Germanica  and  Doryphora 

Decemlineata. 


BY 

WM.  M.  WHEELER. 


r  B.  K:ingBl9V. 

I 


Reprinted  from  The  Journal  of  Morphology,  Vol.  III.,  No.  2,  Sept.,  18S9. 


BOSTON : 

GINN  AND  COMPANY. 


t.  ’rOx^  RWCV 


S9l.3i 
V/Sb  i 


THE  EMBRYOLOGY  OF  BLATTA  GERMANICA 
AND  DORYPHORA  DECEMLINEATA. 

WILLIAM  M.  WHEELER. 

The  following  study  of  the  development  of  the  cockroach  and 
potato-beetle  was  taken  up  during  the  summer  of  1887.  On 
the  suggestion  of  Dr.  W.  Patten,  without  whose  stimulating 
friendship  and  assistance  the  work  would  not  have  been  under¬ 
taken,  I  began  with  Blatta  as  a  form  calculated  to  help  me  to  a 
knowledge  of  the  fecundative  changes  in  the  Hexapod  egg.  Dr. 
Patten  kindly  placed  at  my  disposal  the  results  of  his  own  work 
on  Blatta^  in  the  form  of  much  carefully  prepared  material  and 
some  figures,  which  I  have  incorporated  in  Plate  III.  (Figs.  43, 
45  to  47,  52). 

Later  I  concentrated  my  attention  on  Doryphoray  which  I 
found  to  be  a  much  more  profitable  object  of  study  than  Blatta, 
especially  as  far  as  the  more  advanced  stages  were  concerned. 
Thus  it  happens  that  my  remarks  on  oogenesis  and  fecundation 
are  more  complete  in  Blatta,  while  my  account  of  the  germ-layers 
and  subsequent  stages  is  carried  into  greater  detail  in  Doryphora. 

I  have  seen  fit  to  treat  of  both  insects  as  nearly  as  possible 
under  single  headings,  instead  of  describing  them  independently 
in  two  chapters,  because  they  differ  strikingly  in  all  the  details 
of  development,  while  their  main  ontogenetic  features  are  as 
strikingly  similar.  By  running  both  descriptions  as  nearly  as 
possible  in  parallel  lines,  the  contrasting  details  are  made  more 
salient,  while  the  general  remarks  may  be  taken  up  at  intervals 
and  not  reserved  en  masse  till  the  end  of  the  paper. 

Preparation. 

There  are  three  common  species  of  Blattidce  in  Southeastern 
Wisconsin  :  P eriplaneta  orientalis  (Linn.),  Platamodes  tmicolor 
(Scud.),  and  Blatta  germanica  (Linn.).  The  first  and  the  last 
occur,  as  is  well  known,  about  houses ;  the  second  is  abundant 
under  the  bark  of  decaying  logs  and  stumps  in  open  woods. 


542337 


WHEELER. 


[VoL.  nr. 


292 

Periplaneta  ormitalis  oviposits  from  April  to  August ;  Blatta 
ge7'fnanica,  at  all  times  during  the  year.  The  oothecae  of  Plata- 
modes  may  be  collected  in  great  numbers  where  the  insects 
abound.  After  many  futile  attempts  to  open  the  oothecae  of 
Periplaneta  and  P latamodes  without  injuring  the  ova,  I  limited 
my  study  to  Blatta^  the  egg-capsules  of  which  may  be  easily 
opened  by  the  method  given  below.  By  careful  treatment  of 
the  thick-walled  capsules  of  Periplaneta  and  P latamodes  with  a 
sufficiently  strong  solution  of  sodium  hypochlorite,  it  may  be 
found  possible  to  isolate  the  ova  in  an  uninjured  condition. 

Specimens  of  Blatta  germaiiica  can  be  obtained  at  all  times 
of  the  year  from  places  which  they  haunt,  and  with  very  little 
attention  will  live  long  in  confinement.  The  male  is  long  and 
narrow,  tapering  anteriorly  and  posteriorly  ;  the  female  is  much 
broader  and  flatter  and  uses  her  wings  much  less  than  the  male. 

Males  seem  to  endure  the  unfavorable  condition  of  captivity 
much  better  than  females.  When  it  is  desired  to  time  the  eggs, 
the  capsule  cannot  be  detached  from  the  female  without  damage 
till  it  has  been  rotated,  and  during  winter  must  be  kept  under 
a  bell  jar  with  plenty  of  moist  blotting-paper  to  prevent  the 
embryos  shrivelling  from  the  dryness  of  the  air. 

The  ovarian  ova  in  all  stages  up  to  maturity  were  dissected 
out  in  normal  salt  solution  and  hardened  for  fifteen  minutes  in 
Perenyi’s  fluid.  They  were  then  transferred  to  70  per  cent 
alcohol,  which  was  changed  several  times  at  intervals  of  an  hour, 
and  were  finally  preserved  in  alcohol  of  the  same  strength. 
When  stained  with  borax  carmine  and  sectioned,  the  yolk  retained 
none  of  the  red  stain,  while  the  chromatin  of  the  nucleus  shone 
out  as  a  glistening  deep  red  spot.  Perenyi’s  fluid  rendered  the 
chorion  of  the  mature  ovarian  egg  pervious  to  borax  carmine. 

Hardening  in  a  saturated  aqueous  solution  of  corrosive  subli¬ 
mate  gave  good  results  with  young  ovarian  eggs. 

Oviposited  eggs  were  killed  by  placing  the  capsules  in  water 
slowly  heated  to  8o°-90°  C.  The  two  lips  of  the  crista  of  the 
capsule  were  then  separated  by  the  aid  of  fine  forceps,  and  pieces 
of  the  walls  torn  away,  till  the  eggs  could  be  easily  pushed  out 
of  the  compartments  formed  by  their  choria. 

The  ova  thus  isolated  were  either  transferred  directly  through 
35  per  cent  (10  min.)  to  70  per  cent  alcohol,  or  they  were  left  for 
15  minutes  in  Kleinenberg’s  picrosulphuric  acid,  and  after  re- 


No.  2.] 


BLATTA  AND  DORYPHORA. 


293 


peated  washing  in  70  per  cent  alcohol,  preserved  in  alcohol  of 
the  same  strength.  Both  methods  gave  equally  good  results. 

Though  I  have  succeeded  in  dissolving  the  chitin  of  the 
ootheca  with  sodium  hypochlorite,  the  method  of  tearing  off 
the  walls  after  heating  to  80°  C.  gave  such  satisfactory  results 
that  I  adhered  to  it  throughout  my  work. 

I  have  found  Grenacher’s  borax  carmine  in  every  way  the 
most  expedient  and  reliable  staining  fluid.  Eggs  and  embryos 
up  to  the  time  when  the  cuticle  develops  were  stained  before 
imbedding  in  paraffine ;  the  sections  of  other  embryos  were 
stained  on  the  slide  after  attaching  them  with  Mayer’s  albumen 
fixative. 

The  clusters  of  bright  yellow  eggs  of  the  potato-beetle  (Dory- 
phora  decemlineata,  Say)  may  be  found  on  the  under  surfaces  of 
the  leaves  of  the  potato-plant  during  the  whole  summer,  as  the 
insect  is  polygoneutic. 

The  beetles,  frequently  found  copulating,  may  readily  be  kept 
in  confinement,  and  will  deposit  their  eggs  in  the  typical  flat 
clusters  on  the  walls  of  any  box  or  vessel  in  which  they  are  kept. 
As  I  commenced  collecting  material  late  in  the  season,  I  did  not 
keep  the  insects  in  confinement  till  they  oviposited,  but  collected 
the  eggs  from  the  plant.  It  was  found  convenient  to  cut  out 
the  piece  of  the  leaf  to  which  the  egg-cluster  was  attached  and 
to  keep  it  by  itself  during  the  process  of  preparation,  as  all  the 
eggs  of  a  cluster  are  in  almost  exactly  the  same  stage  of  de¬ 
velopment. 

Beautiful  results  in  preparation  were  obtained  by  heating  the 
eggs  to  80°  C.  for  10  minutes  in  Kleinenberg’s  picrosulphuric 
acid  (with  3  volumes  of  water)  and  preserving  in  70  per  cent 
alcohol. 

By  this  process  the  envelopes,  which  in  the  fresh  egg  adhere 
closely  to  the  yolk,  dilate  and  stand  off  from  the  surface  of  the 
egg,  and  except  in  the  very  youngest  stages  can  be  rapidly  and 
easily  removed  with  the  dissecting  needles. 

A  great  number  of  eggs,  heated  to  65°  C.  only,  or  hardened 
in  cold  Perenyi’s  fluid,  corrosive  sublimate  or  simple  alcohol, 
proved  to  be  useless,  as  the  envelopes  adhered  firmly  to  the  sur¬ 
face  of  the  yolk. 

The  hot  picrosulphuric  acid  fixes  the  cells  of  the  embryo  in  a 
most  satisfactory  manner ;  enough  details  of  the  karyokinetic 


294 


WHEELER. 


[VOL.  III. 


figures  being  preserved  to  enable  one  to  recognize  dividing  nuclei 
at  a  glance.  All  the  eggs  were  imbedded  after  treating  with 
clove  oil,  in  paraffine  melting  at  about  55°  C.  The  somewhat 
gummy  yolk  cut  without  any  tendency  to  crumble,  and  perfect 
series  of  sections  were  obtained  without  the  slightest  difficulty. 

Staining  on  the  slide  with  borax  carmine  gave  beautiful  results 
in  all  stages,  but  was  resorted  to  only  in  young  eggs,  the  vitel¬ 
line  membranes  of  which  did  not  stand  off  from  the  surface  of 
the  yolk,  and  in  advanced  embryos  which  had  developed  the 
larval  cuticle.  To  save  time,  embryos  in  other  stages  were 
stained  before  imbedding,  after  removing  the  egg-envelopes. 
As  much  as  possible  of  the  borax  carmine  was  extracted  with 
acidulated  35  per  cent  alcohol. 

Historical. 

The  cockroaches  have  long  been  favorite  objects  of  morpho¬ 
logical  study.  Easily  obtained  at  all  seasons  of  the  year,  of 
convenient  size  for  dissection,  and  being  but  slightly  modified 
descendants  of  the  oldest  insects  of  geological  time,  they  com¬ 
bine  qualifications  which  make  them  especially  interesting  and 
valuable  to  the  morphologist.  Thus  we  find  that  no  less  than 
twenty  investigators  have  sought  material  for  anatomical  and 
embryological  study  in  the  common  species  of  Blattidce.  I  will 
mention  only  those  who  have  treated  of  the  oogenesis  and  ontog¬ 
eny  of  Blatta  and  Periplaneta. 

Rathke  (42)  was  the  first  to  publish  an  account  of  the  devel¬ 
opment  of  Blatta  germanica.  Brandt  (6)  made  a  study  of  the 
ovarioles  of  Periplafieta.  Huxley’s  (22)  account  of  the  general 
anatomy  of  the  same  insect  contains  some  valuable  remarks  on 
the  ovaries.  Kadyi  (23)  has  given  us  a  condensed  account  of 
the  oviposition  and  micropyles  of  Pe7'ipla7teta  orientalis.  Patten 
(38)  in  1884  published  a  preliminary  note  on  the  development 
of  Blatta.  He  observed  that  the  first  and  second  maxillae  are 
at  first  triramous,  and  made  some  remarks  on  the  heart  and  on 
the  peculiar  organs  developed  from  the  appendages  of  the  first 
abdominal  somite.  Stuhlmann  (45)  treated  of  the  degeneration 
of  the  germinal  vesicle  in  Periplaneta.  During  the  same  year 
(1886)  also  appeared  Miall  and  Denny’s  work  (32)  on  the  anat¬ 
omy  of  Periplanetay  containing  Nusbaum’s  brief  embryological 


No.  2.] 


BLATTA  AND  DORYPHORA. 


295 


description  of  Blatta^  carelessly  written  and  with  figures  often 
grossly  inaccurate. 

Blochmann’s  important  paper  (5),  announcing  the  discovery 
of  polar  globules  in  Blatta  germanica  and  two  other  insects, 
appeared  in  1887.  The  description  of  the  eggs  of  Blatta  is  suc¬ 
cinct  and  perfectly  accurate. 

Of  late,  Cholodkowsky  (lo)  has  published  a  preliminary  paper 
on  the  formation  of  the  entoderm  in  Blatta. 

Doryphora  decemlineata  has  not  been  investigated  heretofore 
from  an  ontogenetic  standpoint.  It  is  surprising  that  so  com¬ 
mon  an  insect,  and  one  whose  eggs  present  such  advantages  for 
embryological  study,  should  have  been  overlooked.  The  favored 
Coleopteron  of  embryologists  has  always  been  Hydrophilus^  and 
it  is  certain  that  the  water-beetles  {Hydrophilidce  and  Dytis- 
cidce)  are  much  less  modified  forms  than  the  leaf-beetles  (Ckry- 
somelidcB)y  to  which  Doryphora  belongs. 

Nevertheless  the  development  of  several  Chrysomelids  has 
been  studied  more  or  less  incompletely. 

Packard  (35)  made  a  brief  study  of  Gastropkysa  coeruleipennis^ 
and  Melnikow  and  Kowalevsky  (26)  studied  Donacia. 

More  exhaustive  was  the  attention  bestowed  on  Lina  by 
Graber  (i  5),  who  published  his  account  in  the  second  volume  of 
his  text-book  on  insects  (1877).  As  would  be  expected  from 
their  close  systematic  affinities,  Doryphora  and  Lina  differ  but 
slightly  in  their  development. 

Ovaries  and  Oviposition. 

Blatta. 

The  ovaries  of  Blatta  are  flattened,  broadly  spindle-shaped 
masses  slung  in  trabecular  connective  tissue,  continuous  with 
the  peritoneum.  Each  ovary  consists  of  from  14  to  26  ovarioles, 
or  egg-tubes  opening  into  the  oviduct.  The  latter  extends 
backwards  towards  the  median  longitudinal  axis  of  the  body, 
and  after  joining  the  oviduct  from  the  opposite  ovary  opens  into 
the  broad  and  short  vagina.  Besides  the  tubular  colleterial 
glands  the  vagina  carries  on  its  dorsal  face  near  the  proximal 
ends  of  the  oviduct  a  thick-walled  sac,  the  spermatheca. 

The  ovariole  has  the  structure  typical  in  insects.  The  fol¬ 
licles  in  all  stages  of  formation  are  inclosed  by  the  membrana 


296 


WHEELER. 


[VOL.  III. 


propria  in  the  form  of  a  tube  tapering  to  capillary  caliber  at  its 
upper  extremity,  which  is  attached  to  the  pericardium.  The 
lumen  of  the  capillary  portion  is  filled  with  protoplasm  in  which 
numerous  small  nuclei  are  imbedded.  This  portion  of  the  ova- 
riole  constitutes  the  germarium.  The  small  nuclei  differentiate 
at  the  lower  end  of  the  germarium,  on  the  one  hand  into  ova, 
which  fill  the  widening  lumen  of  the  tube ;  and  on  the  other, 
into  flattened  epithelial  cells,  which  line  the  inner  surface  of  the 
tube,  and  form  the  follicles  inclosing  the  ova.  There  are  about 
ten  distinct  ova  in  an  ovariole,  the  lowest  being  the  largest,  and 
the  most  apical  the  smallest  and  most  indistinct,  those  interme¬ 
diate  regularly  diminishing  in  size  towards  the  apex.  In  Peri- 
plafieta  there  are  about  three  times  as  many  ova  in  an  ovariole ; 
but  there  are  only  eight  ovarioles  in  an  ovary.  The  lower  ova 
in  both  species  are  oval,  and  are  surrounded  on  all  sides  by  the 
epithelium,  which  has  grown  in  between  the  separate  eggs  to 
complete  the  follicles. 

The  follicular  epithelium  (Fig.  5)  is  composed  of  large,  flat, 
polygonal  cells,  with  lenticular  nuclei  which  present  an  intri¬ 
cately  coiled  chromatin  filament  and  a  nucleolus  of  unusual  struc¬ 
ture.  The  latter  consists  of  an  irregular  mass,  not  stainable  in 
carmine  or  methylgreen,  and  is  regarded  as  plastin  by  Car- 
noy  (9),  who  describes  and  figures  very  similar  nucleoli  in  the 
egg-follicles  of  Gryllotalpa.  The  mass  of  plastin  incloses  a 
smaller  mass  of  chromatin,  or  at  least  of  a  substance  which  does 
not  differ  in  its  reactions  from  the  chromatin  of  the  coiled  fila¬ 
ment  in  the  same  nuclei.  In  eggs  taken  from  the  ovaries  just  be¬ 
fore  maturity,  when  the  epithelium  is  still  firmly  attached  to  the 
underlying  chorion,-  almost  all  of  the  nuclei  will  be  found 
rapidly  dividing.  Pieces  of  the  epithelium  from  eggs  of  differ¬ 
ent  ages  were  examined  in  normal  salt  solution,  in  methylgreen 
acetate  held  for  a  moment  in  the  fumes  of  osmic  acid,  in  RabPs 
chromformic  acid,  in  Zaccharias’  acetic  osmic  acid ;  but  no  traces 
of  an  achromatic  spindle,  or  of  a  regular  arrangement  of  the 
nuclear  filament,  so  characteristic  of  karyokinesis  could  be 
observed.  I  therefore  conclude  that  we  have  here  a  case  of 
akinesis  or  direct  division.  This  conclusion  is  further  strength¬ 
ened  by  the  observation  that  the  nucleolus  divides  first  (Fig.  5  ^), 
and  the  nuclear  wall  is  constricted  during  division,  an  occur¬ 
rence  exceedingly  rare  in  kinetic  nuclei,  where  the  nuclear  wall 


No.  2.] 


BLATTA  AND  DORYPHORA. 


297 


disappears  in  all  but  a  few  of  the  recorded  cases  both  in  plants 
and  animals.  Moreover,  the  two  daughter  nuclei  are  frequently 
very  unequal  in  size. 

The  chorion  (Fig.  i)  is  a  thin,  chitinous  membrane  smoothly 
covering  the  surface  of  the  egg.  In  surface  view  it  appears  to 
be  finely  granular,  the  finest  granules  being  arranged  in  large, 
more  or  less  regularly  hexagonal  areas,  which  are  bounded  by 
narrow,  dark  spaces  containing  somewhat  larger  though  less 
dense  granules.  Each  of  the  hexagonal  areas  is  secreted  by 
one  of  the  polygonal  epithelial  cells  described  above.  It  is  only 
in  cross-section  that  the  true  structure  of  the  chorion  becomes 
apparent.  According  to  Blochmann  (5), — and  my  observations 
coincide  with  his,  — the  chorion  consists  of  two  chitinous  laminae 
kept  in  close  apposition  by  means  of  numerous  minute  trabe¬ 
culae,  or  pillars.  It  is  the  ends  of  these  pillars  seen  in  surface 
view  that  look  like  granules.  In  the  spaces  between  the  hex¬ 
agonal  areas,  the  trabeculae  are  more  scattered  and  individually 
thicker  than  those  of  the  hexagons.  Hence  these  lines  on  the 
chorion  seem  covered  with  larger  and  more  scattered  granules. 
When  pieces  of  the  dry  chorion  are  immersed  in  glycerine  and 
immediately  examined  under  the  microscope,  the  thick  liquid 
may  be  seen  entering  the  spaces  between  the  hexagonal  areas, 
passing  along  them  in  obedience  to  the  laws  of  capillarity,  and 
then  slowly  creeping  from  them  on  both  sides  into  the  adjacent 
hexagonal  areas  between  their  denser  trabeculae.  I  have  also 
observed  that  the  structure  of  the  chorion  of  the  ripe  egg  is 
most  distinct  in  cross-section  at  the  pole  directed  towards  the 
germarium.  Here  the  two  laminae  seen  at  00  in  Fig.  4  separate 
somewhat,  and  the  connecting  trabeculae  become  longer  and 
more  distinct. 

I  have  not  been  able  to  trace  the  formation  of  the  micropyles 
in  Blatta  gerrnanica.  Their  structure  is  easily  demonstrated. 
They  are  scattered  over  a  quadrant  of  the  upper  hemisphere 
where  the  beautiful  hexagonal  pattern  of  the  chorion  gives 
away  to  an  even  trabeculation  (Fig.  2).  The  micropyles  are 
wide-mouthed,  very  oblique,  funnel-shaped  canals  perforating 
the  chorion  (Fig.  2  a,  b).  The  apertures  of  the  funnels 
appear  under  a  low  power  as  clear,  oval  spots,  the  long  axes  of 
which  are  parallel  to  the  long  axis  of  the  egg.  These  perfora¬ 
tions  are  scattered  over  the  micropylar  area,,  sometimes  in  clus- 


298 


WHEELER. 


[VOL.  III. 


ters,  sometimes  singly.  With  a  higher  power  the  tube  of  each 
funnel  is  clearly  visible  as  a  thin  canal  which  dilates  rapidly 
into  the  large  oval  aperture  on  the  outer  face  of  the  chorion. 
The  narrow  tube  is  sometimes  fully  as  long  as  the  large  orifice. 
The  micropylar  perforations  are  all  directed  from  the  germa- 
rium  to  the  vaginal  pole  of  the  egg.  Hence  a  line,  the  hypo¬ 
thetical  path  of  the  spermatozoon,  drawn  through  one  of  these 
oblique  micropyles,  and  continued  into  the  egg,  would  strike 
the  equatorial  plane.  The  female  pronucleus,  as  we  shall  see 
further  on,  moves  in  this  plane. 

The  micropyles  of  Periplaneta,  first  described  by  Kadyi  (23), 
do  not  differ  essentially  from  those  of  Blatta.  In  Periplaneta 
the  hexagonal  pattern  is  continuous  over  the  micropylar  area. 
The  large  micropyles,  which  are  more  yellowish  than  the  sur¬ 
rounding  chorion,  are  thick  walled  and  not  regularly  oval,  as  in 
Blatta,  but  oblong  or  subpentagonal.  The  tube  is  shorter  and 
terminates  on  an  hexagonal  area.  Sometimes  the  micropyles 
are  very  close  together  and  seem  to  overlap. 

I  have  repeatedly  sought  in  vain  for  a  vitelline  membrane  in 
the  eggs  of  Blatta.  Blochmann  (5)  had  no  better  success.  It 
may  exist,  but  it  must  be  exceedingly  thin  and  inseparably  glued 
to  the  inner  lamina  of  the  chorion. 

The  colleterial  glands  of  Blatta  are  like  those  which  Huxley 
(22)  and  Kadyi  (23)  have  described  for  Periplaneta,  a  number  of 
long,  blind  tubes  opening  into  the  vagina.  They  furnish  the  ma¬ 
terial  for  the  capsule,  viz.  :  chitin  and  large  crystals  of  calcium 
oxalate.  In  Blatta  these  glands  are  glistening  white  till  the  time 
of  oviposition  approaches,  when  they  assume  a  yellow  tint,  and 
the  octahedral  crystals  are  seen  imbedded  in  a  viscid  sub¬ 
stance  which  fills  their  lumina.  This  viscid  substance  is  sol¬ 
uble  in  potassium  hydrate,  and  is  consequently  not  chitin. 
When  excreted  to  form  the  ootheca,  it  slowly  hardens,  deepens 
in  color,  and  becomes  insoluble  in  potassium  hydrate.  Light 
has  nothing  to  do  with  this  change,  which  is  possibly  produced 
by  the  oxygen  in  the  air.  It  is  the  same  change  which  is 
undergone  by  the  cuticula  of  the  insect  itself  immediately 
after  ecdysis. 

I  have  made  a  few  observations  on  the  oviposition  of  Blatta 
germanica,  similar  to  those  published  by  Kadyi  (23)  on  Peri¬ 
planeta  orientalis. 


No.  2.J 


BLATTA  ABtD  DORYPHORA. 


When  about  to  form  the  capsule,  the  female  Blatta  closes  the 
genital  armature,  and  the  two  folds  of  the  white  membrane 
which  lines  the  oothecal  cavity  close  vertically  in  the  middle 
line.  Then  some  of  the  contents  of  the  colleterial  glands  are 
poured  into  the  chamber  and  bathe  the  inner  surface  of  the 
posterior  wall.  The  first  egg  glides  down  the  vagina  from 
the  left  ovary,  describes  an  arc,  still  keeping  its  germarium  pole 
uppermost,  after  having  pressed  the  micropylar  area  against  the 
mouth  of  the  spermatheca,  passes  to  the  right  side  of  the  back 
of  the  chamber,  and  is  placed  perpendicularly  two-thirds  to  the 
right  of  the  longitudinal  axis  of  the  insect’s  body.  The  next 
egg  comes  from  the  right  ovary,  describes  an  arc  to  the  oppo¬ 
site  side  of  the  body,  decussating  with  the  path  of  the  .first  egg, 
and  is  placed  completely  on  the  left  side  of  the  median  line. 
The  third  egg  comes  from  the  left  ovary,  and  is  made  to  lie 
completely  on  the  right  side  of  the  median  line  :  and  so  the 
process  continues ;  the  ovaries  discharging  the  eggs  alternately, 
and  each  egg  describing  an  arc  to  the  opposite  side  of  the  cap¬ 
sule.  In  females  killed  during  oviposition  each  oviduct  will  be 
found  distended  with  eggs,  often  two  or  three  end  to  end,  in¬ 
creasing  the  length  and  breadth  of  the  lumen  to  an,  abnormal 
degree.  Gentle  pressure  of  the  female’s  abdomen  between  the 
thumb  and  finger  will  sometimes  cause  the  insect  to  oviposit 
a  few  eggs,  the  paths  of  which  can  be  seen  to  decussate. 

The  oothecal  chamber  soon  becomes  too  small  to  contain  all 
the  constantly  accumulating  eggs,  so  the  anal  armature  opens 

and  allows  the  end  of  the  capsule  to  project.  A  raised  line,  the 

0 

impression  of  the  edges  of  the  white  membrane,  runs  down  the 
end  of  the  capsule.  The  last  egg  deposited  comes  from  the  right 
ovary  and  lies  two-thirds  on  the  left  and  one-third  to  the  right 
of  the  median  line.  Thus  the  first  and  last  eggs  laid  lie  with 
their  greater  bulk  on  opposite  sides  of  the  median  vertical  plane 
of  the  capsule,  and  serve  to  commence  and  close  the  series  and 
round  off  both  ends  of  the  capsule.  Owing  to  their  being 
crowded  up  against  the  walls  of  the  capsule,  they  acquire  quite 
a  different  shape  from  the  remaining  symmetrically  and  alter¬ 
nately  deposited  ova.  They  develop  normally,  however,  the 
embryo  appearing  on  the  inner  obtuse  edge.  As  soon  as 
the  last  egg  is, laid,  a  further  discharge  from  the  colleterial 
glands  spreads  over  the  vaginal  or  anterior  wall  of  the  cavity, 


300 


WHEELER. 


[VoL.  iir. 


and  becomes  evenly  continuous  with  the  secretion  which  has 
before  been  spread  over  the  back  and  the  sides  of  the  capsule  by 
the  white  membrane.  When  the  anterior  end  of  the  capsule  is 
examined,  the  escutcheon-shaped  vaginal  opening  is  found  to 
have  left  its  impression  even  to  the  delicate  wrinkles  into  which 
the  surrounding  cuticula  was  thrown  by  the  closing  of  the  ori¬ 
fice.  This  end  of  the  capsule  is  white,  gradually  shading  into 
the  brown  of  the  opposite  end. 

The  crista,,  a  cord-like  ridge  running  the  full  length  of  the 
dorsal  surface  of  the  capsule,  is  a  thick-walled  tube,  either  half 
of  which  is  formed  by  the  edge  of  the  side  walls  of  the  capsule 
split  into  two  laminae  (Fig.  3  0^,  0^).  The  rhythmical  clasping  of 
the  three  pairs  of  palpi,  which  guard  the  vaginal  opening,  is 
registered  in  an  exquisite  pattern  on  the  inner  face  of  either 
half  of  the  crista. 

The  canal  is  filled  with  a  vacuolated  substance  (Fig.  3  ep) 
which  at  first  sight  resembles  the  yolk  of  the  egg,  but  when 
examined  more  closely  is  seen  to  have  quite  a  different  struc¬ 
ture  and  origin.  In  the  egg  ready  to  leave  its  follicle  the  epi¬ 
thelium  is  much  thickened  at  the  germarium  pole  into  a  bicon¬ 
vex-lens-shaped  cap  (Fig.  4),  the  cells  of  which  are  not  flat  like 
those  on  the  other  portions  of  the  egg,  but  long,  columnar,  and 
more  or  less  curved.  The  two  laminae  of  the  chorion  spread 
apart  beneath  this  cap  and  dilate  into  a  pear-shaped  sac  divided 
up  into  numerous  polygonal  chambers  by  delicate  chitinous 
partitions  (Fig.  4  b).  While  the  egg  is  leaving  the  follicle,  the 
epithelium  at  the  lower  pole  is  loosened  from  the  chorion,  and  * 
the  egg  protrudes  into  the  oviduct.  As  it  advances,  the  epithe¬ 
lium  is  rolled  back  and  doubled  up  in  folds  till  it  is  freed  from 
the  chorion  as  far  as  the  cap.  Then  it  breaks,  letting  the  egg 
pass  into  the  oviduct  with  the  thick  cap  of  cells  firmly  attached. 
The  egg  is  placed  in  the  capsule,  and  the  cap  comes  to  lie  in 
the  crista,  filling  its  lumen.  The  large  nuclei  degenerate,  and 
soon  entirely  disappear,  the  protoplasm  becomes  dry  and  vacuo¬ 
lated,  and  finally  transformed  into  the  yolk-like  mass  described 
above.  This  substance  probably  serves  as  a  cement  to  keep 
the  lips  of  the  crista  in  contact  till  separated  by  the  emerging 
larvae.  Thus  a  small  portion  of  the  follicular  epithelium,  that 
portion  which  corresponds  to  the  nourishing  cells  in  other 
insect  ovaries,  is  deposited  with  the  egg.  To  my  knowledge 


No.  2.] 


BLATTA  AND  DORYPHORA. 


301 

this  has  not  been  observed  in  any  other  arthropod  eggs  hereto¬ 
fore  described,  excepting  Miisca  (Bruce,  7).  The  pear-shaped 
dilatation  of  the  chorion  is  directly  over  the  head  of  the  future 
embryo  and  hatching  insect,  and  is  possibly  more  easily  rup¬ 
tured  or  dissolved  than  the  surrounding  chorion. 

Of  40  capsules  examined  for  the  purpose  of  noting  which 
ovary  sent  out  the  first  and  which  the  last  egg,  32  com¬ 
menced  with  the  right  egg  (from  the  left  ovary)  and  closed 
with  the  left  egg  (from  the  right  ovary),  six  capsules  commenced 
and  closed  with  the  right,  and  one  commenced  and  closed  with 
the  left  egg.  Evidently  the  32  were  normal ;  in  the  insects 
which  deposited  the  six,  one  of  the  ovarioles  was  probably 
either  atrophied  or  wanting,  though  the  perfectly  alternate 
arrangement  of  the  eggs  in  the  capsule  was  in  nowise  inter¬ 
rupted  on  this  account.  The  two  remaining  egg-capsules  were 
small  and  abnormal. 

The  number  of  eggs  in  a  capsule,  far  from  being  constant  in 
Pei'iplajieta  orientalis,  is  even  more  fluctuating  in  Blatta  ger- 
manica.  In  34  capsules  counted  the  average  number  was  about 
40,  the  least  number  '28,  and  the  greatest  58.  The  number 
varies  in  different  localities  and  is  doubtless  dependent  on  the 
food  of  the  female  insect.  In  several  capsules  obtained  where 
amylaceous  food  was  abundant  the  average  was  much  higher 
than  in  a  much  greater  number  of  capsules  obtained  from  a 
place  where  fatty  food  was  the  only  diet. 

The  above  description  of  the  oviposition  of  Blatta  gernia7iica 
probably  applies  to  most  species  of  the  Blattidce.  But  this 
species  differs  from  Periplaneta  and  probably  many  other  forms, 
in  rotating  the  capsule,  a  process  now  to  be  described.  As 
soon  as  the  last  egg  has  passed  the  vagina  and  been  placed  in 
the  capsule,  the  latter  begins  to  rotate  on  its  longitudinal  axis, 
till  the  crista  has  described  one-fourth  of  a  cylinder  to  the  right. 
The  capsule  is  now  in  a  horizontal  position  having  its  greater 
transverse  diameter  parallel  with  the  corresponding  transverse 
diameter  of  the  insect’s  body.  The  abdomen  contracts  during 
oviposition,  and  its  end  comes  to  lie  anterior  to  the  tips  of  the 
wings,  so  that  the  broad  ends  of  the  latter  hide  and  protect  the 
protruding  end  of  the  capsule.  The  rotation  requires  about  a 
day.  In  one  case  a  female  kept  the  capsule  in  a  vertical  position 
for  two  weeks,  apparently  from  some  inability  to  revolve  it.  In 


WHEELER. 


[VoL.  riT. 


362 

all  other  cases  the  capsule  was  regularly  turned  to  the  right, 
never  to  the  left. 

The  female  Peripla7ieta  orientalis  drops  the  capsule  soon  after 
its  completion,  with  predilection  in  some  food  supply,  such  as 
flour  or  meal.  Blatta  gennanica  carries  it  for  about  a  month, 
then  drops  it  shortly  before  the  hatching  of  the  larvae.  Some 
writers  claim  that  the  parent  assists  the  young  in  escaping  from 
the  capsule,  but  I  have  proved  by  many  experiments  that  the 
expanding  and  struggling  of  the  young  insects  are  amply 
sufficient  to  separate  the  feebly  united  lips  of  the  crista.  Such 
an  instinct  in  the  female  would  be  of  use  only  if  the  lips  of  the 
capsule  could  not  be  opened  by  the  young.  Taschenberg  (46) 
claims  that  the  female  regularly  lays  only  one  capsule  and  dies 
soon  after  its  deposition.  My  observations  on  fifty  females, 
whose  wings  were  clipped  as  soon  as  they  had  formed  their  first 
capsule,  have  convinced  me  that  they  certainly  lay  two  perfect 
capsules  as  a  rule,  and  possibly  more,  in  the  course  of  the 
year. 

As  will  be  seen  from  the  preceding  account,  it  is  a  very  easy 
matter  to  orient  the  eggs  of  the  capsule,  to  tell  just  what 
position  any  oothecal  egg  held  in  the  ovary,  or  just  what  position 
any  egg  in  the  ovary  will  hold  in  the  capsule.  The  germarium 
pole  of  the  ovarian  egg  lies  just  beneath  the  crista  after  oviposi- 
tion.  The  concave  side  of  the  curved  sausage-shaped  ovarian 
egg  is  turned  to  the  wall  of  the  capsule,  and  its  convex  face,  on 
which  the  embryo  will  appear  with  its  head  towards  the  crista,  is 
turned  to  the  interior  of  the  capsule  and  faces  the  corresponding 
surface  of  the  opposite  egg.  The  micropyles  which  pointed 
away  from  the  germarium  are  on  the  convex  face  of  the  ova¬ 
rian,  and  on  the  inner  face  of  the  oothecal  egg,  and  point  down¬ 
wards  away  from  the  crista. 

The  Development  of  the  Egg  to  the  Formation  of  the 

Blastoderm. 

Blatta. 

The  oothecal  egg  of  Blatta^  like  the  ovarian  egg,  is  glistening 
white.  The  latter  has  been  described  as  sausage-shaped,  but 
the  pressure  exerted  by  the  eggs  on  one  another  in  the  capsule 
alters  their  original  form  very  considerably.  They  assume  the 


No.  2.] 


BLATTA  A.VD  DORYPHORA. 


303 


shape  of  half  an  elliptical  disc,  the  short  axis  of  which  is  to  the 
long  axis  as  2  is  to  3,  with  a  thickness  one-sixth  of  the  short 
axis  (Figs.  36  and  37).  The  egg  is  about  3  mm.  long,  i  mm. 
broad  and  J  mm.  thick.  Its  volume  is  therefore  almost  a  cubic 
millimeter.  The  cephalic  end  (Fig.  36  c),  recognized  by  its 
evenly  rounded  contour,  is  immediately  beneath  the  crista  of 
the  capsule.  The  opposite  or  caudal  end  (Fig.  36  s)  is  dis¬ 
tinguished  by  a  slight  sinus.  The  ventral  face  is  traversed 
by  a  keel  which  runs  from  the  cephalic  to  the  caudal  extremity 
and  is  most  pronounced  a  short  distance  below  the  middle  of 
the  egg.  The  dorsal  surface  is  flat  and  evenly  curved  antero- 
posteriorly.  Cross-sections'  of  the  ovum  are  consequently  pen¬ 
tagonal  (Fig.  40).  Thus  the  eggs  can  be  easily  oriented,  and 
they  have  a  great  advantage  over  spherical  or  even  oval  ones, 
in  that  all  the  earliest  developmental  changes  can  be  traced 
directly  to  their  relationship  with  the  parts  of  the  future 
embryo.  This  is  of  the  highest  importance  in  the  early  stages. 

It  will  have  been  observed  that  a  complete  reflexion  of  the 
egg  takes  place  during  oviposition  as  the  concave  face  of  the 
curved  ovarian  egg  becomes  the  convex  back  of  the  oothecal 
egg,  and  the  convex  micropylar  face  of  the  former  becomes  the 
straight,  carinated,  ventral  face  of  the  latter. 

Yolk.  —  I  have  not  been  able  to  observe  a  passage  of  fol¬ 
licular  epithelial  cells  into  the  egg  to  form  yolk  by  their  dis¬ 
integration,  as  has  been  described  by  Will  (51)  in  Nepa  and 
Notonecta,  and  by  Ayers  (i)  in  Qicanthus.  There  is  only  one 
layer  of  cells  in  the  follicular  epithelium  of  Blatta^  and  as  this 
persists  till  after  the  chorion  is  completed,  no  migration  of  nu¬ 
clei  into  the  yolk,  or  even  disintegration  of  nuclei  at  the  surface 
of  the  egg,  is  observable.  All  of  the  yolk  in  Blatta  (excepting 
those  portions  derived  from  the  germinal  vesicle  i*)  is  secreted  by 
the  protoplasm  of  the  epithelial  cells,  not  as  yolk,  but  as  sub¬ 
stances  which  are  taken  up  by  the  growing  ovule,  and  again 
secreted  in  the  form  of  the  bodies  to  be  described  presently. 
During  this  process  of  yolk-secretion,  the  epithelial  cells  re¬ 
main  intact,  their  slow  disintegration  not  taking  place  till  after 
oviposition,  when  their  compacted  and  yellowish  remains  have 
assumed  the  appearance  so  suggestive  of  the  corpora  lutea  of  the 
Mammalia. 

The  yolk  of  the  eggs  of  Blatta  has  been  studied  by  Patten 


304 


WHEELER. 


[VOL.  III. 


{38)  and  Blochmann  (5).  The  former  has  described  the  physical 
structure ;  the  latter,  the  peculiar  bilateral  distribution  of  the 
yolk  elements.  Though  my  researches  have  revealed  only 
a  few  new  facts  concerning  the  yolk,  I  will  give  them  in  their 
entirety. 

In  the  young  ovarian  egg,  0.5  mm.  long,  the  nucleus  is 
surrounded  by  granular  protoplasm  in  which  no  yolk  has 
developed.  In  eggs  i  to  2  mm.  long  the  yolk  consists  of  two 
kinds  of  bodies  :  transparent  fat  globules  of  various  sizes,  and 
a  translucent  albuminous  substance  broken  up  into  distinctly 
outlined  masses  which  are  polygonal  in  form  from  mutual 
pressure.  Beneath  the  follicular  epithelium  is  a  layer  of  small 
albuminous  masses  enveloping  the  interior  yolk.  This  is  at  a 
time  when  the  large  nucleus  is  disintegrating.  By  the  time  the 
egg  has  reached  its  full  size,  the  yolk  has  assumed  the  highly 
differentiated  structure  best  studied  in  the  oothecal  egg. 

The  yolk  of  a  fresh  mature  egg  crushed  between  the  slide  and 
the  cover  glass  in  its  own  liquids,  or  in  normal  salt  solution,  shows 
an  abundance  of  different  sized  highly  refractive  oil  globules, 
and  a  greater  number  of  distinctly  outlined  albumen  spheres, 
which  are  polygonal  in  the  intact  egg  where  mutual  pressure 
prevents  them  from  taking  on  the  spherical  shape  which  they 
seem  to  be  continually  striving  to  assume.  These  albumen 
spheres  are  thin-walled  sacs  full  of  a  thin  liquid  in  which  float 
multitudes  of  small,  irregular  granules.  Sometimes  small  oil  glob¬ 
ules  are  enclosed.  That  the  contents  of  these  sacs  is  a  thin  liquid 
is  proved  by  the  exceedingly  active  Brownian  movement  of  the 
irregular  granules,  a  movement  which  is  certainly  constant  in 
almost  all  these  vitelline  bodies  in  the  living  egg.  But  these 
bodies  are  far  from  being  alike  in  structure.  In  some  few  the 
granules  are  very  minute,  closely  packed,  and  exhibit  no 
Brownian  movement ;  in  others  the  granules  are  large  and  dis¬ 
tinctly  irregular,  few  in  number,  and  possessed  of  a  tendency  to 
unite  in  flakes  which  hang  suspended  in  the  thin  liquid  filling 
the  sacs.  In  the  great  majority  of  these  bodies,  however,  the 
mean  between  these  two  extremes  in  structure  is  maintained.  It 
is  probable  that  the  granular  forms  arise  from  the  dense  homo¬ 
geneous  bodies  by  chemical  decomposition.  This  decomposi¬ 
tion,  which  is  accomplished,  as  I  said  above,  in  the  ripe  ovarian 
egg,  takes  place  in  different  parts  of  the  egg  in  different  degrees, 


No.  2.] 


BLATTA  AND  DORYPHORA, 


305 


thus  bringing  about  the  peculiar  bilateral  structure  first  noticed 
by  Patten  and  subsequently  described  by  Blochmann.  In 
hardened  eggs  the  granules  have  the  appearance  of  nets  with 
larger  or  smaller  meshes  (Figs.  23,  24,  etc.).  This  structure 
is  best  seen  in  cross-sections  of  eggs  hardened  in  alcohol  or 
picrosulphuric  acid. 

For  the  distribution  of  the  above-described  yolk-elements  in 
the  egg  I  cannot  do  better  than  quote  Blochmann’s  succinct  and 
accurate  account.  He  says  :  “  Die  Hauptmasse  des  Dotters 
ercheint  auf  dem  Querschnitt  [see  my  Fig.  40]  etwa  als  gleich- 
schenkeliges  Dreieck,  das  seine  unpaare  (kurze)  Seite  nach 
aussen,  seine  Spitze  der  Mitte  zukehrt.  Diese  Dottermasse 
besteht  aus  durch  gegenseitigen  Druck  polygonal  abgeplatteten 
Korpern,  deren  Umrisse  besonders  an  den  peripheren  Par- 
tien  deutlich  sind,  wahrend  sie  in  der  Mitte  des  Eies  mehr 
Oder  weniger  zusammenfliessen.  In  den  peripheren  Regionen 
zeigen  diese  Korper  eine  feine  Granulirung  ihrer  Sub- 
stanz,  wahrend  sie  im  Innern  des  Eies  vollstandig  homogen 
erscheinen.  Sie  farben  sich  bei  der  gewohnlichen  Behand- 
lung  mit  Boraxkarmin  ziemlich  intensiv.  Umgeben  wird  diese 
Dottermasse  von  einer  Zone  [my  Eigs.  39  and  40]  von  eben- 
falls  polygonalen  Elementen,  die  jedoch  iiberall  deutlich  unter 
sich  und  von  den  Elementen  (der  Hauptmasse  des  Dotters) 
sich  abgrenzen.  Sie  zeigen  eine  grobnetzige  Struktur  und  far¬ 
ben  sich  mit  Boraxkarmin  nur  ganz  wenig.  Diese  Zone  des 
Dotters  hat  ihre  grosste  Dicke  liber  der  Spitze  des  von  dem 
anderen  gebildeten  Dreiecks  und  nimmt  nach  den  Seiten  zu  ab, 
um  in  der  Mitte  der  Riickseite  wieder  etwas  an  Machtigkeit 
zu  gewinnen.  Dieser  blasse  Dotter  bildet  also  einen  kontinu- 
irlichen  Mantel  um  den  anderen.  (Es)  finden  sich  langs 
der  ganzen  Eioberflache  zwischen  die  blassen  Dotterkorper 
eingelagert  einzelne  starker  farbbare  Elemente  des  Dotters 
(der  Hauptmasse)  oder  auch  kleine  Gruppen  von  solchen. 

Unregelmassig  durch  beide  Dotterarten  finden  sich 
zahlreiche  grossere  und  kleinere  Fetttropfen,  die  an  den 
Praparaten  als  Hohlraume  erscheinen,  da  das  Eett  durch  die 
Behandlung  mit  Chloroform,  Terpentinol,  etc.  aufgelost  wird” 
(my  Eigs.  23,  24,  etc.). 

The  ventral  thickening  of  the  “continuous  mantel  ”  of  granular 
yolk  polyhedra,  which  surrounds  the  wedge-shaped  central  mass 


3o6 


WHEELER. 


[VOL.  III. 


of  translucent  polyhedra,  is  a  pre-arrangement  for  the  reception 
of  the  blastoderm,  and  more  particularly  the  future  embryo. 
The  vital  activities,  which,  in  forming  the  embryo,  finally  be¬ 
come  centred  on  the  ventral  face  of  the  egg,  find  the  yolk  here 
already  sufficiently  decomposed  to  admit  of  easy  metabolism  into 
protoplasm.  Later,  in  eggs  with  advanced  embryos  on  their 
ventral  faces,  the  granular  yolk  has  again  become  transparent 
and  homogeneous  in  extensive  masses  which  arise  from  the  con¬ 
fluence  of  many  of  the  polyhedral  bodies.  I  differ  from  Bloch- 
mann  in  maintaining  that  the  granular  yolk  is  originally  derived 
from  the  homogeneous  variety.  He  was  led  to  infer  the  oppo¬ 
site  process  from  his  study  of  the  yolk  in  ants’  eggs. 

Protoplasm, — In  young  ovarian  eggs,  0.25  to  0.5  mm  long, 
the  cytoplasm  is  finely  granular.  The  deutoplasm  begins  to 
accumulate,  and  by  the  time  the  egg  has  become  i  mm  long, 
the  above-described  vitelline  bodies  and  oil  globules  have  de¬ 
veloped  in  such  numbers  as  to  reduce  the  protoplasm  to  an 
exceedingly  delicate  net.  In  the  mature  egg  the  KeiinhatU 
described  for  so  many  insects’  eggs  is  a  layer  so  thin  that  it  is 
just  perceptible  on  the  centre  of  the  dorsal  surface  and  at 
the  cephalic  pole  (Fig.  4pr).  This  protoplasmic  layer  is  full  of 
what  I  shall  call  Blochmann’s  corpuscles.  They  are  minute  rod¬ 
shaped  bodies  so  numerous  in  the  surface  protoplasm  as  to  make 
it  appear  reticulate.  They  look  like  bacillar  micro-organisms, 
stain  deeply,  and  according  to  Blochmann,  who  is  probably  right 
in  thinking  that  they  play  an  important  role  in  the  development 
of  the  egg,  multiply  by  transverse  division  (like  Bacteria). 
Weismann  was  the  first  to  note  these  bodies  in  Diptera  in  1863. 
Blochmann  (3,  4,)  called  attention  to  them  in  1884  and  1886,  in 
the  eggs  of  Camponotiis  and  Formica,  and  in  1887  in  Blatta,  Peri- 
planeta,  Pieris,  Musca,  and  Vespa.  In  the  three  last  genera  the 
bodies  are  spherical.  In  several  of  Stuhlmann’s  (45)  figures, 
these  bodies  are  prominent.  In  Blatta  I  have  found  them  wher¬ 
ever  the  peripheral  layer  of  protoplasm  is  perceptibly  thickened, 
especially  surrounding  the  polar  globules  on  the  middle  of  the 
dorsal  surface  (Figs.  14,  1 5,  and  16),  and  at  the  cephalic  end  of  the 
egg  (Fig  4 p'T).  They  seem  to  be  made  of  more  rigid  material 
than  the  protoplasm  in  which  they  are  embedded,  for  they  pro¬ 
trude  as  very  minute  prickles  from  the  surfaces  of  eggs  hardened 
in  Perenyi’s  fluid.  I  have  not  been  able  to  trace  out  their  der¬ 
ivation  and  ultimate  destiny. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


307 


As  the  granular  yolk  polyhedra  of  the  ventral  portion  of  the 
egg  are  much  smaller  than  the  homogeneous  ones  of  the  in¬ 
terior,  there  is  more  protoplasm  in  the  ventral  part  of  the  egg. 
The  internal  portions  crumble  very  easily  in  sectioning,  and 
I  am  hence  inclined  to  think  that  very  little  or  no  protoplasm 
extends  in  between  these  bodies  from  the  periphery.  Thus  we 
see  that  the  distribution  of  the  protoplasm,  as  well  as  the  yolk, 
is  in  accordance  with  the  position  of  the  future  embyro.  Later, 
when  cells  appear  in  the  egg,  their  amoeboid  cytoplasm  con¬ 
sists  of  evenly  granular  protoplasm,  faintly  stainable  in  borax 
carmine. 

Karyog7'apJiy  of  tJie  Egg.  —  In  studying  the  changes  in  the 
nucleus,  it  has  proved  to  be  impossible  to  preserve  the  finest 
cytological  details,  since  the  ovarian  eggs  of  BlattUy  like  the 
eggs  of  the  Orthoptera  in  general,  are  not  easily  sectioned  un¬ 
less  hardened  in  Perenyi’s  fluid,  as  the  yolk  is  exceedingly 
friable.  Perenyi’s  and  Kleinenberg’s  fluids  do  not  preserve  the 
karyokinetic  figures  perfectly,  but  cause  the  loops  of  chromatin 
to  fuse  in  masses.  On  the  other  hand,  the  achromatic  spindles 
are  often  beautifully  distinct.  Any  discrepancy  between  Bloch- 
mann’s  figures  of  the  polar  globules  and  mine,  is  probably  to 
be  attributed  to  a  difference  in  the  use  of  reagents. 

In  young  and  transparent  ova  taken  from  the  middle  of  the 
ovariole,  the  nucleus  is  seen  to  have  reached  the  acme  of  its 
development  in  volume.  It  is  a  large  spherical  body,  more 
highly  refractive  than  the  surrounding  cytoplasm.  Its  fluid 
contents,  the  karyoplasm,  is  distinctly  separable  into  two  sub¬ 
stances,  a  liquid  karyochylema  and  an  achromatic  (plastin  T) 
reticulum.  In  the  meshes  of  dhe  latter  is  suspended  a  third 
substance,  the  deeply  stainable  chromatin,  in  one  large  and  sev¬ 
eral  smaller  masses,  or  in  several  small  irregular  particles  de¬ 
rived  from  the  former  by  disintegration.  The  nuclear  wall  is 
very  distinct.  Whether  it  is  a  membrane  or  merely  a  peripheral 
inspissation  of  the  karyoplasm  is  uncertain.  When  ovarioles 
are  treated  with  P'ol’s  picrochromic  acid  or  Merkel’s  chromplat- 
inum  solution,  the  whole  nucleus  contracts  and  leaves  a  menis- 
coid  cavity  between  itself  and  the  cytoplasm,  though  it  still 
preserves  its  distinct  and  evenly  spherical  contour. 

In  all  the  following  stages  the  egg,  having  become  opaque, 
must  be  studied  in  sections,  and  the  history  of  the  nucleus  con- 


308 


WHEELER. 


[VoL.  iir. 


structed  from  a  series  of  isolated  observations.  While  the  yolk 
is  accumulating,  the  nucleus  becomes  amoeboid,  leaves,  its  posi¬ 
tion  at  the  centre  of  the  egg,  and  travels  to  the  surface.  The 
egg  is  at  this  time  very  slightly  curved,  and  the  nucleus  always 
passes  to  the  centre  of  the  concave  side.  This  amoeboid 
nucleus  is  seen  in  Fig.  6  just  after  reaching  the  surface.  It  is 
still  large,  and  does  not  stain  more  deeply  than  the  surrounding 
yolk  bodies.  Its  long  axis  is  parallel  to  the  long  axis  of  the 
egg.  The  chromatin,  in  large  masses  in  the  younger  egg,  has 
been  reduced  to  numerous  granules  of  varying  size,  still  recog¬ 
nizable  as  chromatin  because  staining  deeply  in  borax  carmine. 
The  pseudopodia  are  now  drawn  in,  and  the  nucleus  becomes 
spheroidal.  Soon  the  face  in  contact  with  the  surface  becomes 
cup-shaped,  and  round  masses  of  a  homogeneous  substance  indis¬ 
tinguishable  from  the  surrounding  yolk  bodies  fill  the  cavity 
(Figs.  7  and  8  b).  In  this  stage  the  nucleus  is  probably  giving 
off  the  “maturation  spheres”  (Reifungsballen),  which  Stuhlmann 
saw  given  off  from  the  nucleus  of  so  many  insects’  eggs  (notably 
Lepidoptera)  ;  Will  (51)  also  describes  this  process  in  Hemiptera. 
In  many  insects  Stuhlmann  found  that  these  spheres  differed 
from  the  surrounding  yolk  bodies  in  power  of  refraction,  etc. 
The  concavity  (Einbuchtung)  of  the  nucleus  in  Blatta  often 
contains  several  of  these  spheres.  Figure  8  represents  such  a 
nucleus  seen  from  the  surface,  the  plane  of  section  being  at 
right  angles  to  the  plane  of  section  in  Figs.  6  and  7.  Here  the 
concavity  is  composed  of  separate  cavities  which  have  fused. 
The  maturation  spheres,  after  their  escape  from  the  nucleus, 
mingle  with  the  yolk  bodies.  Above  the  orifice  in  Fig.  8, 
at  pn,  is  a  small  body,  denser  and  more  refractive  than  the 
surrounding  plasma.  I  think  it  corresponds  to  Stuhlmann’s 
paranucleolus.  In  its  centre  is  a  deep  red  body,  probably  a 
granule  of  chromatin.  In  Fig.  9  is  seen  a  nucleus  containing  two 
paranucleoli,  /;/,  destitute  of  the  central  granule.  I  cannot  say 
what  becomes  of  these  bodies.  They  do  not  appear  in  all  nuclei 
{confer  Fig.  7),  and  are  probably  evanescent  structures  formed 
and  again  disintegrated  during  the  mysterious  process  of  nuclear 
degeneration.  In  Fig.  9  granules  of  chromatin,  most  numerous 
near  the  point  «,  are  scattered  through  the  karyoplasm.  The 
disintegration  of  the  nucleus  by  giving  off  the  maturation 
spheres  progresses  till,  when  the  egg  is  about  2  mm.  long,  it  is 


No.  2.] 


BLATTA  AND'  DORVPffORA. 


5(3g 


reduced  to  a  small,  somewhat  crescentic  body,  which,  unless  its 
position  has  been  marked,  is  easily  mistaken  for  a  yolk  body, 
as  it  in  no  way  differs  from  the  surrounding  yolk  in  ability  to 
take  up  coloring-matters.  Hence  it  happens  that  many  investi¬ 
gators  have  supposed  the  insect  egg  to  become  enucleate  at  this 
time.  The  resemblance  between  a  yolk  polyhedron  and  the 
remains  of  the  germinal  vesicle  is  greatly  increased  by  the  gran¬ 
ular  contents  of  both.  In  the  germinal  vesicle  these  granules 
are  the  comminuted  remains  of  the  large  masses  of  chromatin  so 
conspicuous  in  the  young  egg ;  in  the  yolk  bodies  they  are  albu¬ 
minous  substances  destined,  like  the  other  yolk  materials,  to 
become  food  for  the  protoplasm.  At  the  very  spot  where  the 
nucleus  degenerated,  viz.  at  the  middle  of  the  concave  side  of 
the  egg,  there  appears  in  eggs  almost  mature  (2. 5-2.8  mm.  long), 
a  cluster  of  numerous  chromatin  granules,  which  I  believe  to  be 
the  same  as  those  in  the  germinal  vesicle,  grown  more  conspic¬ 
uous  by  aggregation.  Stuhlmann  has  figured  several  bodies 
like  my  Fig.  10,  representing  this  aggregation  which  has  pro¬ 
gressed  considerably  in  the  centre.  These  chromatin  granules 
are  probably  uniting  to  form  filaments  preparatory  to  karyo- 
kinesis.  The  aggregation  takes  place  in  such  a  way  that  in 
eggs  treated  with  Perenyi’s  fluid  a  narrowly  oblong  mass  of 
chromatin  is  formed,  an  appearance  undoubtedly  due  to  the 
fusion  of  the  separate  filamentous  loops.  The  oblong  mass  is 
represented  in  Fig.  ii.  When  the  outlying  granules  have  been 
added  to  it,  and  the  achromatic  fibrillae  have  made  their  appear¬ 
ance,  the  first  polar  spindle  in  the  metakinetic  stage  is  completed 
(P'ig.  12).  The  axis  of  the  spindle  is  directed  at  right  angles  to 
the  surface  of  the  egg,  which  is  now  mature  though  still  enveloped 
in  the  follicular  epithelium.  In  Fig.  13  the  equatorial  plate 
has  divided  transversely,  whether  by  longitudinal  fissure  of  the 
individual  loops  or  not,  I  am  unable  to  say,  and  the  two  masses 
of  chromatin  are  on  their  way  to  the  poles  of  the  spindle. 
Arrived  at  the  poles  the  masses  become  spherical,  and  the 
achromatic  spindle  fades  away,  while  the  outer  sphere  of  chro¬ 
matin  surrounded  by  a  mass  of  protoplasm  full  of  Blochmann’s 
corpuscles  is  almost  constricted  off  from  the  egg  to  form  the  first 
polar  globule.  While  this  is  taking  place  the  egg  is  being  freed 
from  all  its  epithelium,  except  the  cap  at  the  cephalic  pole,  im¬ 
pregnated  and  placed  in  the  capsule.  The  daughter  nucleus  of  the 


510 


WHEELER. 


[VoL.  nr. 


first  polar  spindle  remaining  within  the  egg  now  forms  another 
spindle  directed  like  the  first.  This  is  seen  in  Fig.  14.  The 
blade  of  the  microtome  has  somewhat  raised  the  loosely  at¬ 
tached  first  polar  globule,  the  protoplasm  of  which  is  seen  to 
contain  many  of  Blochmann’s  corpuscles.  The  karyokinesis" 
of  the  second  spindle  progresses  in  the  same  manner  as  the 
first  and  the  second  polar  globule  is  given  off,  also  surrounded 
by  bacillar  protoplasm.  The  portion  of  the  nucleus  which 
becomes  the  female  pronucleus,  is  not  seen  in  Fig.  15,  where 
only  the  two  polar  globules  are  represented,  as  it  appears  in  the 
next  section.  Figure  16  is  from  a  surface  view  of  the  polar  glob¬ 
ules.  By  comparison  with  Fig.  i^pgl  i  it  will  be  seen  that  they 
are  lenticular.  Here  the  female  pronucleus  appears  as  a  more 
indistinct  body  (because  out  of  focus)  lying  between  the  polar 
globules.  The  polar  globules  which  I  have  been  able  to  find  in 
many  eggs  taken  from  the  capsules  while  they  were  still  vertical 
in  the  genital  armature  (about  6  to  12  hours  after  the  begin¬ 
ning  of  oviposition)  lie  in  the  middle  of  the  convex  dorsal  wall 
of  the  oothecal  egg.  They  do  not  divide  subsequently  to  form 
what  Weismann  (48)  calls  secondary  polar  globules,  but  soon 
disintegrate.  In  eggs  about  one  day  old  their  remains  may  be 
found  as  an  amorphous  granular  mass,  lying  just  beneath  the 
chorion  and  entirely  separated  from  the  egg. 

The  female  pronucleus  increases  considerably  in  size  before 
leaving  the  surface  of  the  egg  (Fig.  179  p^t)-  This  increase  is 
gradual  but  constant  as  it  makes  its  way  through  the  dense 
yolk  of  the  interior  of  the  egg  to  the  apex  of  the  isosceles  tri¬ 
angle  of  homogeneous  yolk  abutting  on  the  back  part  of  the 
granular  ventral  yolk.  It  is  on  this  journey  that  the  female 
pronucleus  meets  the  male  pronucleus  formed  from  a  spermato¬ 
zoon  which  has  entered  the  egg  by  one  of  the  funnel-shaped 
micropyles  on  the  upper  ventral  face. 

Though  I  have  succeeded  in  throwing  a  little  more  light  on 
the  copulation  of  the  pronuclei  than  Blochmann,  I  cannot  regard 
my  observations  as  completely  satisfactory.  The  process  must 
be  studied  in  Arthropod  eggs  with  more  evenly  compact  yolk 
than  the  eggs  of  Orthoptera,  the  numerous  cracks  and  fissures 
in  which  render  the  observation  of  delicate  internal  processes 
exceedingly  difficult  if  not  impossible.  Moreover,  the  copula¬ 
tion  of  the  pronuclei  in  Blntta  is  hurried  through  very  rapidly. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


311 

In  more  than  a  hundred  eggs  which  I  sectioned  from  capsules  6 
to  24  hours  old  I  found  the  greater  number  in  the  polar  globule 
stage  and  all  the  remainder,  with  two  exceptions,  in  the  stages 
just  before,  during  and  after  the  division  of  the  cleavage  nu¬ 
cleus.  In  the  two  exceptions  which  I  describe  and  figure  I 
found  what  I  take  to  be  the  pronuclei.  The  arrows  in  the 
figures  point  in  the  direction  of  the  long  axis  of  the  egg,  only 
the  circumnuclear  portions  of  which  are  represented. 

In  Fig.  19  the  female  pronucleus  is  about  one-third  the  dis¬ 
tance  from  the  cephalic  end  instead  of  being  in  the  middle  of 
the  egg  where  the  polar  globules  are  formed.  It  is  no  further 
from  the  dorsal  surface  than  the  female  pronucleus  in  Fig.  17, 
but  is  much  larger.  Hence  I  believe  it  has  travelled  up  along 
the  dorsal  surface  to  meet  the  male  pronucleus  (Fig.  19  pn)^ 
which  has  advanced  through  almost  the  whole  dorsoventral 
diameter  of  the  egg.  The  female  pronucleus  exhibits  the  usual 
coiled  chromatin  filament.  The  male  pronucleus  is  granular,  of 
somewhat  irregular  outline,  and  surrounded  by  vacuolated  pro¬ 
toplasm.  It  is  rather  deeply  stainable  in  borax  carmine.  In 
this  Fig.  19  the  pronuclei  are  of  very  different  size,  and  not¬ 
withstanding  the  male  pronucleus  has  passed  through  three- 
fourths  of  the  dorsoventral  diameter,  a  distance  of  f  mm.,  it  has 
not  increased  much  in  volume  when  compared  with  the  mass  of 
the  long  though  attenuated  head  of  the  spermatozoon  from 
which  it  originated.  No  astral  radiation  could  be  seen  surround¬ 
ing  these  pronuclei.  In  Fig.  20  we  have  what  I  take  to  be  the 
two  pronuclei  conjugating  The  smaller  presumably  male  portion 
of  the  compound  or  cleavage  nucleus  is  larger  than  the  male  pro¬ 
nucleus  in  Fig.  19.  The  place  of  union  is  about  the  middle  of 
the  homogeneous  yolk,  i.e.  about  one-third  of  the  dorsoventral 
diameter  from  the  back  of  the  egg.  The  nuclei  of  Fig.  19 
would  probably  have  fused  at  a  point  near  the  dorsal  surface 
one-third  the  distance  from  the  cephalic  to  the  caudal-  pole,  but 
the  paths  of  these  two  nuclei  were  undoubtedly  aberrant.  My 
observations  on  single  female  pronuclei  and  cleavage  nuclei 
found  at  various  points  along  the  median  dorsoventral  diameter 
lead  me  to  conclude  that  the  middle  of  the  homogeneous  yolk  is, 
the  normal  point  of  conjugation.  I  have  represented  (Fig.  18) 
another  female  pronucleus  from  near  the  point  of  fusion  because 
its  structure  is  different  from  that  of  the  other  female  pronuclei 


312 


WHEELER. 


[VOL.  III. 


figured.  Its  chromatin  is  limited  to  a  few  filaments  ;  whereas 
in  Fig.  ig  9  pi  we  have  a  long  and  intricately  wound  coil. 

The  cleavage-  nucleus  continues  the  course  begun  by  the 
female  pronucleus.  Figure  21  represents  it  when  it  has  reached 
the  middle  of  the  dorsoventral  axis.  The  cytoplasm  surround¬ 
ing  this  nucleus,  which  is  drawn  from  an  egg  hardened  in 
picrosulphuric  acid,  shows  an  astral  radiation.  The  cytoplasm 
passes  by  insensible  degrees  into  the  surrounding  homogeneous 
yolk  substance.  It  is  as  if  the  female  pronucleus  when  it  left 
the  polar  globules  took  a  little  mass  of  the  protoplasm  abun¬ 
dant  at  that  part  of  the  egg’s  surface  and  travelled  with  it, 
making  it  convert  the  yolk  into  protoplasm  during  the  journey. 

As  soon  as  the  cleavage  nucleus  reaches  the  front  edge  of 
the  mass  of  homogeneous  yolk,  or  has  advanced  a  very  short 
distance  further  into  the  granular  yolk,  it  stops  and  begins 
to  increase  in  size  till  it  becomes  a  clear  vesicle  (Fig.  22)  in 
which  the  chromatin,  broken  into  irregular  fragments,  lies  scat¬ 
tered  through  the  finely  granular  contents.  The  nuclear  wall 
grows  fainter  and  disappears.  The  large  spindle  now  appears, 
and  the  typical  process  of  karyokinesis  is  carried  on  (Fig. 
23).  The  polar  axis  of  the  spindle  was  in  all  but  one  of  the 
many  cases  in  which  I  observed  the  division,  parallel  to  the  long 
axis  of  the  egg.  In  the  exceptional  case  (possibly  an  abnormal 
egg)  it  was  parallel  to  the  dorsoventral  diameter.  Even  before 
the  division  of  the  nucleus  commences,  the  polar  axis  of  the 
future  spindle  is  foreshadowed,  as  it  were,  in  the  shape  of  the 
granular  (amoeboid)  cytoplasm,  which,  as  may  be  seen  in  Fig. 
23  cp^  is  elongated  in  a  direction  parallel  to  the  cephalocaudal 
axis  of  the  egg.  The  karyokinetic  process  employed  in  this 
and  the  subsequent  divisions  is  typical.  Soon  after  the  nuclear 
wall  again  makes  it  appearance  the  two  nuclei  with  the  cytoplasm 
which  surrounds  them  separate  about  one-fifth  the  longitudinal 
diameter  of  the  egg,  and  then  prepare  for  the  next  division. 
The  polar  axes  of  the  two  daughter  spindles  now  formed  are  very 
nearly,  at  right  angles  to  the  polar  axis  of  the  mother  nucleus. 

In  all  the  first  divisions  the  perfect  isochronism  of  the  differ¬ 
ent  stages  in  the  different  nuclei  is  striking.  Fig.  24  represents 
two  spindles  from  an  egg  containing  four  nuclei  in  exactly  the 
same  (metakinetic)  stage.  In  this  figure,  taken  from  a  longitu¬ 
dinal  section,  one  of  the  spindles  (c)  has  its  axis  at  right  angles 


No.  2.] 


BLATTA  AATD  DORVPHORA. 


313 


to  the  longitudinal  and  also  to  the  dorsoventral  axis  of  the  egg. 
The  loops  forming  the  equatorial  plate  are  readily  seen  and 
counted.  Ten  in  number,  each  probably  attached  to  an 
achromatic  fibril,  they  are  arranged  in  such  a  manner  that 
seven  form  a  circle  inclosing  the  remaining  three. 

The  divisions  of  these  four  products  of  the  cleavage  nucleus 
continue  till  about  60  to  80  cells  have  been  formed,  scattered 
irregularly  through  the  granular  ventral  yolk.  The  axes  of 
the  spindles  are  inclined  in  various  directions,  and  nothing  in¬ 
dicates  an  early  differentiation  into  cells  destined  on  the  one 
hand  to  remain  in  the  yolk,  and  on  the  other  to  form  the 
blastoderm. 

The  numerous  amoeboid  cells  next  migrate  to  the  surface  of  the 
egg.  In  Fig.  25,  representing  the  ventral  third  of  a  median  cross- 
section,  two  of  these  cells  have  just  reached  the  surface,  while 
one  is  still  on  its  way.  On  reaching  the  surface  the  cells  first 
become  somewhat  conical,  and  then  gradually  flatten  out.  The 
tension  of  the  cytoplasm  is  so  great  that  the  inclosed  nucleus  is 
forced  to  become  lenticular  (Fig.  26  a).  The  cells  which  have 
reached  the  surface,  and  are  much  scattered  over  the  roof-shaped 
ventral  face  and  the  adjacent  portions  of  the  lateral  faces,  com¬ 
mence  dividing  tangentially,  not  by  karyokinesis,  as  heretofore, 
but  by  akinesis.  Figure  36  represents  the  lateral  surface  view, 
and  Fig.  26  part  of  a  transverse  section  of  an  egg  in  this  stage. 
The  division  of  the  nuclei  which  have  reached  the  surface  is 
very  rapid,  and  compact  colonies  of  cells  of  different  sizes  and 
in  different  stages  of  the  unequally  constricting  process  charac¬ 
teristic  of  akinesis  may  be  seen  embedded  in  amoeboid  masses 
of  protoplasm.  I  have  given  such  a  syncytium  (enlarged  from 
Fig.  36)  in  Fig.  34,  and  two  of  the  dividing  nuclei  from  other 
parts  of  the  same  egg  in  Fig.  35  b,  c.  The  method  of  division 
is  exactly  like  what  was  described  above  for  the  cells  of  the  fob 
licular  epithelium  (Fig.  5),  omitting  the  peculiar  nucleoli  which 
I  have  not  been  able  to  detect  in  these  nuclei.  My  observa-- 
tions  tend  to  show  that  all  the  future  divisions  in  the  formation 
of  the  blastoderm  and  those  subsequently  undergone  by  the 
serosa  are  akinetic,  the  densely  coiled  chromatin  filament  re¬ 
maining  inert,  and  the  division  taking  place  by  a  constriction 
which  often  produces  two  daughter  nuclei  of  very  unequal  size. 
I  emphasize  the  fact  that  these  forms  of  division  could  not  have 


314 


WHEELER. 


[VOL.  III. 


been  produced  by  reagents,  as  these  eggs  were  hardened  in 
picrosulphuric  acid  or  simple  alcohol,  which  in  younger  and 
older  eggs  preserve  the  karyokinetic  figures  of  the  cleavage 
nucleus  and  its  immediate  descendants  with  great  clearness. 

The  nuclei  of  the  small  syncytia  spread  apart  evenly  over  the 
surface  of  the  egg,  which  now  presents  the  appearance  of  Fig. 
37.  The  pseudopodia  of  the  amoeboid  cytoplasmic  masses  run 
together  to  form  a  net.  The  egg  is  now  in  the  blastema  stage 
of  Patten  (38).  The  cells  at  the  surface  are  being  continually 
reinforced  by  cells  migrating  from  the  yolk.  Ever  since  the 
first  division  of  the  cleavage  nucleus  the  nuclei  have  undergone 
a  gradual  and  steady  diminution  in  size,  and  this  progresses  till 
the  formation  of  the  blastoderm  which  takes  place  by  the 
division  of  the  blastema  cells.  In  this  stage  the  yolk  is  covered 
with  a  layer  of  protoplasm,  imperfectly  divided  into  small  cells, 
each  containing  a  lenticular  nucleus,  which  in  turn  contains  two 
very  deeply  stainable  nucleoli  (Phg.  33).  Fig.  27  is  a  trans¬ 
verse  section  through  the  front  of  an  egg  in  the  blastoderm 
stage.  All  the  protoplasm  at  the  surface  of  the  egg  is  carried 
there  by  the  migrating  cells  or  formed  from  the  surface  yolk 
through  their  influence,  as  the  Keimhaiity  so  highly  developed 
in  Doryphora,  as  will  be  seen  further  on,  is  undeveloped  on  the 
ventral,  lateral,  and  all  but  a  very  small  portion  of  the  dorsal 
surface  of  the  egg  of  Blatta  germartica. 

All  the  nuclei,  formerly  in  the  yolk,  probably  7'ise  to  the  stir- 
face  to  fortn  the  blastema  and  reinforce  it  in  its  formation  of 
the  blastoderm.  Before  the  blastoderm  is  completed,  cells  sep¬ 
arate  from  it  and  pass  inwards  to  form  the  yolk  cells,  or 
vitellophags.  The  following  are  my  reasons  for  believing  that 
all  the  products  of  the  cleavage  nucleus  go  to  the  surface.  The 
cleavage  nucleus  cells  have  large  pale  nuclei '  and  distinctly 
amoebiform  cytoplasm,  like  those  in  the  yolk  of  Figs.  25  and  26. 
Subsequently  none  of  these  nuclei  are  to  be  found  in  the  yolk, 
but  in  their  stead  occur  at  greater  or  lesser  distances  from  the 
ventral  and  lateral  faces  small  deeply  stainable  nuclei  of  exactly 
the  same  size  as  the  blastoderm  nuclei,,  not  surrounded  with 
amoebiform  cytoplasm,  but  apparently  melting  their  way  through 
the  yolk,  often  in  the  middle  of  a  dense  yolk  body,  and,  above  all, 
exhibiting  the  same  intimate  structure  as  the  blastoderm  nuclei 
(Fig.  28).  When  treated  with  picrosulphuric  acid,  these  centrip- 


No.  2.J 


BLATTA  AND  DORYPHORA. 


315 


etal  nuclei  show  the  two  highly  refractive  nucleoli  of  the 
surface  cells ;  when  treated  with  alcohol  only,  both  the  surface 
and  yolk  nuclei  exhibit  the  same  closely  wound,  deeply  stainable 
chromatin  filament.  Such  exact  similarity  in  size,  shape,  and 
minute  structure  is  very  strong  evidence  in  favor  of  community 
of  origin.  Still  I  have  not  been  able  to  find  an  egg  without 
nuclei  in  the  yolk,  notwithstanding  I  sectioned  many  eggs  in 
the  blastoderm  stage.  I  am  inclined  to  think  that  such  a  stage 
may  not  occur,  but  that  the  last  of  the  cleavage  nucleus 
products  go  to  the  surface  simultaneously  with  the  passage  in 
the  opposite  direction  of  the  first  blastoderm  products.  Thus 
the  enucleate  yolk  stage  would  be  slurred  over. 

The  time  required  for  the  development  so  far  described  is 
approximately  as  follows  :  The  first  polar  spindle  is  formed  in  the 
ovaries  ;  the  second  polar  spindle  during  oviposition.  Both  polar 
globules  have  been  constricted  off  by  about  the  sixth  hour  from 
the  commencement  of  oviposition.  By  the  end  of  the  first  day 
the  female  pronucleus  has  fused  with  the  male  pronucleus,  and 
the  cleavage  nucleus  thus  formed  has  reached  the  back  of  the 
granular  ventral  yolk.  The  products  of  the  cleavage  nucleus 
are  formed  and  reach  the  surface  by  the  end  of  the  third 
day.  By  the  end  of  the  fourth  day  the  blastema  is  completed. 
During  the  fifth  day  the  blastema  nuclei  proliferate  and 
complete  the  blastoderm  by  the  close  of  the  sixth  day.  The 
development  is,  of  course,  accelerated  by  a  rise  and  retarded  by 
a  fall  in  temperature,  though  not  to  the  extent  observed  in 
many  other  animals. 

Before  passing  over  to  a  description  of  the  early  stages  of 
the  egg  of  Doryphora,  I  will  recapitulate  the  movements  of  the 
nuclei  by  means  of  diagrams  of  a  longitudinal  and  equatorial 
cross-section  of  the  egg  (Figs.  39  and  40).  In  these  diagrams 
a  is  the  cephalic,  b  the  caudal  end,  c  the  ventral,  d  the  dorsal,  0 
the  lateral  surface.  The  shaded  body  p  is  the  homogeneous 
yolk,  the  dotted  portions  r  the  granular  yolk.  The  germinal 
vesicle  starting  from  the  central  point  e  goes  to  the  surface, 
describing  the  path  represented  by  the  line  ef.  Here  it  gives 
off  by  two  successive  divisions  the  two  polar  globules 
and  and  then  as  the  female  pronucleus  turns  back  to 
go  in  the  opposite  direction.  The  line  representing  the  pas¬ 
sage  of  the  germinal  vesicle  to  the  surface  is  really  too  long, 


3i6 


WHEELER. 


[VOL.  III. 


as  this  path  is  travelled  over  when  the  egg  is  much  smaller, 
and  the  distance  from  the  centre  to  the  periphery  much 
less  than  in  ,  the  mature  egg.  The  nucleus  of  the  sperma¬ 
tozoon  entering  the  egg  at  some  point  on  the  ventral  surface 
between  and  r,  probably  at  or  near  h,  advances  through 
the  egg  as  the  male  pronucleus  i  and  fuses  with  the  female 
pronucleus,  coming  from  the  opposite  direction,  at  a  point 
near  k.  The  segmentation  nucleus  passes  on  to  the  point 
/  and  divides  in  the  direction  of  the  anteroposterior  axis  of 
the  egg,  giving  rise  to  the  daughter  nuclei  nun.  By  sub¬ 
sequent  divisions  these  give  rise  to  the  blastema  nuclei,  which 
migrate  to  the  ventral  and  ventrolateral  surfaces  of  the  egg  (;/). 
It  is  by  a  tangential  division  of  the  blastema  cells,  forming  a 
layer  of  much  smaller  cells,  which  creep  around  the  sides  of 
the  egg  and  close  on  the  dorsal  surface,  that  the  blastoderm  is 
completed. 

Doryphora. 

Turning  now  to  Doryphora^  we  find  that  the  ovariole  differs 
from  that  of  Blatta  in  one  particular :  The  terminal  thread  di¬ 
lates  below  into  a  large  oval  chamber  {Endkamnier) ,  the  mem¬ 
branous  wall  of  which  is  closely  packed  with  cells  of  various 
sizes,  containing  nuclei  which  vary  in  size  as  the  inclosing 
cytoplasm  varies  in  volume.  The  nuclei  contain  delicate,  much 
convoluted  chromatin  filaments.  Besides  the  difference  in  size, 
no  other  differences  are  perceptible  between  the  different 
cells. 

At  the  lower  end  of  the  Endkammer  the  differentiation  of 
the  cells  into  ova  and  follicular  epithelium  takes  place.  Careful 
examination  of  many  sections  has  convinced  me  that  none  of 
the  peculiar  phenomena  described  by  Will  (51)  in  the  oogenesis 
of  Nepa  are  to  be  observed  in  Doryphora.  What  I  have  seen 
is  in  perfect  accord  with  Leydig’s  observations  (29).  The  large 
cells  of  the  Endkammer  become  the  ova,  and  the  smaller  cells, 
after  undergoing  a  further  reduction  in  size  by  division,  become 
the  follicular  epithelium. 

In  the  two  upper  follicles  the  ova  resemble  in  every  respect  the 
large  cells  of  the  terminal  chamber,  the  nuclei  retaining  exactly 
the  same  perfectly  spherical  form,  and  the  same  distribution  of 
their  chromatin.  In  the  ovum  of  the  third  follicle  the  chromatin 


No.  2.] 


BLATTA  AND  DORYPHORA. 


317 


has  assumed  a  different  appearance.  It  is  no  longer  distributed  in 
the  long,  even,  much  convoluted  filament,  but  has  broken  up  into 
several  spherules  which  stain  more  deeply.  One  or  more  large 
vacuoles  are  to  be  found  in  each  one  of  these  nucleoli,  or  masses 
of  chromatin,  which  under  a  high  power  seem  to  hang  suspended 
in  the  meshes  of  the  nuclear  reticulum  in  the  same  manner  as 
the  homologous  bodies  of  Blatta.  From  the  time  of  its  first 
formation  till  the  ovum  has  attained  a  considerable  size,  the  fol¬ 
licular  epithelium  is  columnar  with  its  elongate  nuclei  directed  at 
right  angles  to  the  long  axis  of  the  egg.  Later  the  epithelium 
flattens,  and  the  nuclei  become  kidney-shaped,  with  their  long 
axes  tangentially  directed  to  the  surface  of  the  egg  and  their 
hili  directed  inwards. 

The  yolk  first  makes  its  appearance  in  the  form  of  numerous 
granules.  In  no  case  have  I  seen  a  degeneration  of  the  follic¬ 
ular  epithelium  to  form  yolk.  The  large  granular  yolk  spheres 
soon  make  their  appearance.  As  these  bodies  are  much  smaller 
than,  but  in  other  respects  very  similar  to,  those  in  Blatta^  I 
have  given  little  attention  to  their  study.  Though  the  proto¬ 
plasm  is  reduced  to  a  very  delicate  reticulum  by  the  great  ac¬ 
cumulation  of  yolk  spheres,  there  remains  till  the  formation  of 
the  blastoderm,  contrary  to  what  I  have  observed  in  Blatta,  a 
thick  layer  of  finely  and  evenly  granular  protoplasm,  which  en¬ 
velops  the  whole  egg  and  is  equivalent  to  Weismann’s  Keimhaiit, 
though  it  is  present  from  the  first  appearance  of  the  yolk  in  the 
form  of  spheres,  and  does  not  originate  just  before  the  forma¬ 
tion  of  the  blastoderm,  as  in  several  of.  the  insects  studied  by 
Weismann  (47).  The  thick  surface  layer  stains  pale  pink  in 
borax  carmine,  and  is  quite  distinctly  marked  off  from  the  retic¬ 
ulate  yolk-charged  protoplasm  of  the  interior  of  the  egg. 

After  the  nuclear  filament  has  become  metamorphosed  into 
the  spherical  vacuolated  masses  above  described,  the  nucleus 
moves  from  the  centre  of  the  egg  to  the  surface,  travelling  along 
a  line  at  right  angles  to  the  polar  diameter  of  the  egg.  It  thus 
reaches  a  point  midway  between  the  poles,  taking  the  same 
position  as  the  germinal  vesicle  of  Blatta.  During  its  migra¬ 
tion  the  karyoplasm  becomes  amoeboid,  and  except  at  its  outer 
surface  retains  its  irregular  form  even  after  taking  its  position 
right  under  the  follicular  epithelium.  The  outward  directed 
face  becomes  excavated,  the  rest  of  its  surface  remaining  con- 


3i8 


WHEELER. 


[VoL.  iir. 


nected  with  intervitelline  protoplasmic  trabeculae  (Fig.  56  tr). 
The  karyoplasm  of  the  nucleus  is  coarsely  granular.  In  the 
nucleus  figured  only  two  of  the  vacuolated  masses  of  chromatin 
(or  nucleoli,  as  most  writers  call  them)  are  present.  The  larger 
contains  six  vacuoles,  one  of  which  incloses  a  bar-shaped  mass 
of  chromatin  (Fig.  56  nl).  In  the  cavity  of  the  nucleus  a  number 
of  more  or  less  oval  hyaline  masses  are  seen.  They  are  doubt¬ 
less  the  equivalents  of  the  “  maturation  spheres,”  noted  above 
in  Blatta.  Stuhlmann  (45)  has  found  these  same  spheres  in 
the  degenerating  germinal  vesicle  of  LinUy  a  Chrysomelid  allied 
to  Doryphora. 

The  next  stage  found  in  the  decomposition  of  the  germinal 
vesicle  is  represented  in  Fig.  57.  The  karyoplasm  has  become 
confluent  with  the  intervitelline  protoplasm,  and  only  the  chro¬ 
matin  portion  marks  the  spot  where  the  nucleus  reached  the  sur¬ 
face  of  the  egg.  The  larger  yolk  spheres,  formerly  present  even 
in  the  surface  protoplasm,  have  passed  inwards,  and  only  the 
smaller  spheres  still  remain  in  what  is  to  become  the  blastema. 
Soon  these,  too,  retire  further  into  the  egg,  and  the  surface  pro¬ 
toplasm  is  marked  off  from  the  vitelliferous  portion.  The  re¬ 
mains  of  the  nucleoli  are  worthy  of  attention.  The  vacuoles  have 
disappeared,  and  the  glistening  chromatin  has  grown  denser, 
and  stains  very  deeply.  In  the  case  figured,  one  or  two  nucleoli 
have  evidently  broken  into  the  nine  fragments  of  different  sizes 
and  shapes.  The  larger  and  more  peripheral  mass  (Fig.  57  «) 
is  surrounded  by  a  pale  aureole.  Its  size,  position,  and  the 
clear  protoplasm  surrounding  it,  seem  to  point  it  out  as  the  im¬ 
portant  and  permanent  mass  of  chromatin  soon  to  be  converted 
into  the  first  polar  spindle.  The  remaining  eight  fragments 
seem  to  be  leaving  the  surface  and  passing  into  the  yolk,  where 
they  probably  disappear,  as  no  traces  of  them  can  be  found  in 
succeeding  stages. 

In  Fig.  58,  which  represents  the  next  stage  in  the  nuclear 
metamorphosis,  we  find  that  the  mass  n  of  Fig.  57,  which  con¬ 
tained  five  spherical  masses  of  dense  chromatin,  has  become  a 
perfect  oval  nucleus  in  the  resting  stage.  The  chromatin  has 
again  passed  into  the  filamentous  state.  The  surface  layer 
of  protoplasm  is  clearly  developed  and  has  secreted  the  vitelline 
membrane  (Fig.  58  v).  The  chorion,  too  (Fig,  58  ch)y  has  ap¬ 
peared.  The  follicular  epithelium  which  secreted  it  is  omitted 
in  this  and  the  next  figures. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


319 


The  resting  nucleus  just  described  soon  begins  to  divide.  Its 
small  size  prevents  an  accurate  understanding  of  the  peculiari¬ 
ties  of  its  mode  of  division.  Enough  can  be  gleaned  from  Fig. 
59,  however,  to  show  that  the  karyokinesis  is  not  typical  like 
that  described  by  Flemming  (12,  13)  and  Rabl  (41)  for  the 
epithelial  nuclei  of  Salamandra.  No  loops  seem  to  be  present 
in  the  metakinetic  stage,  but  the  chromatin  is  arranged  in  mo- 
niliform  strings,  each  of  which  seems  to  be  applied  full  length 
to  one  of  the  achromatic  fibres  of  the  spindle.  I  have  not  seen 
all  the  stages  in  the  metakinetic  process. 

In  the  next  stage  observed  (Fig.  60)  the  two  masses  of  chro¬ 
matin  resulting  from  metakinesis  have  reached  the  poles  of  the 
spindle,  where  the  fibres  before  apparent  have  become  invisible. 
In  the  equatorial  plane,  however,  the  achromatic  filaments  are 
very  distinct,  being  noticeably  thickened.  This ^  thickening  of 
the  spindle  fibres  to  form  the  dividing  plate  {b)  between  the  two 
cells  is  of  universal  occurrence  in  plants,  as  may  be  seen  from  an 
examination  of  Strasburger’s  (44)  delicate  figures.  It  is,  how¬ 
ever,  not  infrequent  in  Arthropods.  Carnoy  (9)  figures  many 
instances  in  his  “  Cytodierese.” 

Unfortunately  I  have  been  unable,  through  lack  of  material, 
to  trace  the  changes  of  the  nucleus  immediately  following  those 
just  described.  The  ovum  is  deposited  by  the  female  Doryphora 
with  its  nucleus  is  the  stage  represented  in  Fig.  60  (compare 
Blattd).  The  outer  mass  of  chromatin  {p^)  must  be  regarded 
as  the  first  polar  globule.  Probably  the  process  of  forming  the 
second  polar  globe  is  essentially  the  same  as  in  Blatta. 

Though  incomplete,  my  observations  prove,  I  think,  that  polar 
globules  are  also  formed  in  the  Coleoptera,  thus  adding  another 
order  of  insects  to  the  three  in  which  these  interesting  bodies 
were  found  by  Blochmann  (5). 

As  in  Blatta,  the  nuclei  represented  in  Figs.  57  to  60  are  very 
small  and  difficult  to  find  in  the  enormously  larger  eggs. 

Long  before  oviposition  the  eggs  of  Doryphora  have  acquired 
the  dull  orange  color  which  makes  them  so  conspicuous  on  the 
under  surfaces  of  the  leaves  to  which  they  are  glued  by  the 
females.  The  coloring-matter  is  seated  in  the  yolk  bodies,  and 
ultimately  disappears  in  eggs  preserved  in  70  per  cent  alcohol. 
The  eggs  are  most  deeply  colored  in  living  specimens  in  the 
earlier  stages,  and  a  gradual  fading  of  the  color  accompanies  the 


320 


WHEELER. 


[VOL.  III. 


gradual  metabolism  of  the  deeply  colored  yolk  into  pale  yellow- 
protoplasm. 

As  the  yolk  spheres  are  much  smaller  in  Doryphora  than  in 
Blatta,  mutual  pressure  does  not  make  them  so  clearly  polygonal. 

Though  granular,  the  protoplasm,  which  is  spread  in  such  a 
thick  layer  over  the  surface  of  the  yolk,  contains  none  of  the 
bacillar  bodies  so  easily  demonstrated  in  Blatta.  It  is  of  course 
possible  that  in  the  beetle’s  eggs  they  may  be  present,  but  of 
much  smaller  size  and  of  spherical  shape  like  those  found  in  the 
Lepidopteron  Pieris. 

The  chorion  in  Doryphora  is  thick  and  somewhat  leathery, 
though  easily  torn  with  the  dissecting-needles.  It  seems  to  re¬ 
semble  in  every  way  that  of  Lma  as  described  by  Graber  (15). 
The  surface  layer  of  protoplasm  secretes  a  very  delicate  and 
structureless  vitelline  membrane,  which  in  the  younger  stages 
is  closely  applied  to  the  surface  of  the  egg.  Soon  after  the 
formation  of  the  ventral  plate  it  is  loosened  and  stands  off  from 
the  surface.  Besides  some  clear,  irregular  patches  on  the  sur¬ 
face  of  the  chorion  I  have  seen  no  structures  which  could  be 
interpreted  as  micropyles. 

As  there  are  strong  reasons  for  supposing  that  the  cleavage 
nucleus  is  situated  in  the  very  centre  of  the  egg  while  dividing, 
the  copulation  of  the  pronuclei  must  take  place  along  the  radial 
line  joining  the  centre  of  the  egg  to  the  point  where  the  polar 
globules  are  formed.  As  I  possessed  no  eggs  immediately  after 
oviposition,  the  phenomena  of  pronuclear  conjugation,  which 
could  probably  be  more  favorably  studied  in  the  eggs  of  Dory- 
phoi'a  than  in  Blatta,  on  account  of  the  perfect  and  even  sec¬ 
tions  obtainable,  were  not  observed. 

The  first  stages  after  the  one  given  in  Fig.  60  which  I  have 
been  able  to  find  in  my  material  showed  the  division  imme¬ 
diately  following  the  first  and  second  divisions  of  the  cleavage 
nucleus.  As  the  few  nuclei  were  all  near  the  centre  of  the  egg, 
and  as  Graber  has  found  the  cleavage  nucleus  is  the  centre  of 
the  very  similar  eggs  of  the  allied  Lina,  I  feel  justified  in  believ¬ 
ing  that  this  is  the  point  at  which  the  first  division  occurs.  The 
products  of  the  cleavage  nucleus  go  through  karyokinesis,  but 
owing  to  their  much  smaller  size  the  process  is  much  less  dis¬ 
tinct  than  in  Blatta.  In  eggs  stained  with  borax  carmine,  the 
cytoplasm  of  each  cell  appears  as  a  delicate  pink  cloud  among 


No.  2.] 


BLATTA  AND  DORVPHORA. 


321 


the  grayish  yolk  spheres,  and  very  high  powers  are  necessary  to 
detect  the  delicate  rays  of  the  spindle  or  even  the  minute  gran¬ 
ules  of  chromatin  in  the  metakinetic  and  succeeding  stages. 

The  isochronism  among  all  the  nuclei  in  the  different  stages 
of  development  up  to  the  formation  of  the  blastoderm  is  quite 
as  apparent  as  in  Blatta.  Judging  from  the  great  number  of 
eggs  with  resting  nuclei  and  the  very  few  eggs  with  kinetic 
figures,  I  conclude  that  division  takes  place  very  rapidly  and  is 
followed  by  comparatively  long  periods  of  quiescence. 

Fig.  61  represents  one-half  a  cross-section  through  the  equa¬ 
torial  region  of  an  egg  containing  a  number  of  nuclei  in  its  yolk. 
All  the  nuclei  are  resting  and  are  surrounded  by  amoeboid 
masses  of  protoplasm.  These  cells  often  have  the  appearance 
of  being  in  motion,  most  frequently  in  concentric  paths.  The 
nucleus  is  in  the  broader  portion  of  the  comet-shaped  cell, 
which  seems  to  be  advancing  head  foremost.  These  cells  are 
surrounded  by  numerous  minute  vacuoles  that  under  the  low 
power,  with  which  Fig.  61  was  drawn,  appear  like  coarse  granules. 

These  cells  divide  rapidly  and  give  rise  to  many  smaller  cells 
scattered  through  the  whole  yolk.  A  few  enter  the  blastema 
layer  and  begin  to  proliferate  rapidly.  As  soon  as  this  mi¬ 
gration  to  the  surface  has  taken  place,  the  cells  which  have 
remained  in  the  interior,  and  zvJiich  do  7iot  go  to  the  surface,  stop 
dividing  and  take  up  positions  at  short  but  nearly  equal  distan¬ 
ces  apart,  through  the  whole  yolk.  Before  assuming  their  defi¬ 
nite  positions  they  have  multiplied  so  rapidly  that  one  may 
frequently  see  strings  of  three  or  four  cells  (Fig.  62  j/;d). 
Often,  too,  in  this  stage  most  of  the  cells  are  in  pairs,  or,  more 
accurately  speaking,  the  egg  contains  many  binucleate  cells. 

The  nuclei  which  have  entered  the  surface  layer  of  pro¬ 
toplasm  divide  tangentially.  Sometimes  the  axis  of  the  spindle 
is  inclined  at  an  angle  less  than  90""  to  the  radius  of  the  circular 
cross-section,  but  in  no  case  have  I  seen  a  spindle  with  its  axis 
directed  radially.  The  first  divisions  of  the  nuclei,  which  have 
entered  the  blastema,  give  rise  to  an  even  layer  of  cubical  cells. 
By  one  more  division  of  its  constituent  elements  this  blastema 
is  converted  into  the  blastoderm,  which  consists  of  smaller  and 
more  columnar  cells.  Sections  taken  in  all  directions  through 
the  egg  show  the  blastoderm  to  be  of  even  thickness  over  the 
whole  surface  (Fig.  63). 


322 


WHEELER, 


[VOL.  III. 


General  Remarks. 
a.  Nuclear  Continuity . 

It  will  be  seen  from  the  above  descriptive  paragraphs,  that  I 
maintain  that  a  portion  of  the  chromatin  of  the  insect  egg 
visibly  survives  in  the  decomposition  of  the  germinal  vesicle  and 
can  be  traced  through  the  divisions  resulting  in  the  formation 
of  the  two  polar  globules  into  the  cleavage  nucleus  and  its 
descendants.  This  conclusion,  which  has  always  been  held  by 
careful  investigators  of  the  transparent  ova  of  lower  forms,  has 
been  seriously  questioned  of  late  by  two  workers,  Stuhlmann 
(45)  and  Henking  (20).  The  former,  after  investigating  a  num¬ 
ber  of  Arthropod  eggs  in  a  superficial  manner,  comes  to  the 
conclusion  that  a  stage  exists  in  the  ontogeny  of  the  ovum  when 
no  traces  of  a  nucleus  can  be  demonstrated.  Henking  not 
only  indorses  this  view,  but  describes  in  Opilio  what  are  cer¬ 
tainly  the  products  of  division  of  the  cleavage  nucleus  as  arising 
dc  novo  in  different  parts  of  the  egg. 

As  Blochmann  (5)  has  pointed  out  the  errors  into  which  both 
investigators  have  fallen  with  far  greater  force  than  I  can  bring 
to  bear  on  the  subject,  I  will  not  increase  the  length  of  my 
paper  by  entering  into  a  detailed  account  of  their  observations. 
Blochmann’s  beautiful  researches  on  the  early  stages  of  the  egg 
have  proved  beyond  a  doubt  that  the  Hexapoda  conform  to  the 
fecundative  processes  and  method  of  polar-globule  formation 
observed  in  other  animals. 

• 

That  there  is  no  7noment .when  the  nucleus  ceases  to  exist  as 
a  nucleus  seems  to  me  to  be  proved  by  my  Fig.  ii,  where  the 
remains  of  the  nuclear  wall  are  still  present  while  the  spindle 
is  forming.  True,  the  wall  is  absent  in  the  younger  stage.  Fig. 
10,  and  Fig.  ii  may  represent  an  exceptional  case  in  which  the 
wall  has  persisted  longer  than  is  usual,  but  it  proves,  neverthe¬ 
less,  that  the  matter  composing  the  nucleus  does  not  diffuse 
through  the  protoplasm  and  ultimately  recombine  to  form  the 
nuclei  which  give  rise  to  the  blastoderm,  as  Henking  (20)  would 
have  us  believe.  The  nuclear  wall  is  known  to  persist  in 
another  Arthropod  till  after  the  commencement  of  spindle  forma¬ 
tion.  I  quote  the  following  from  the  recent  work  of  Weis- 
mann  and  Ischikawa  (49) :  “  Auch  in  den  kleinen  und  dotterlosen 


No.  2.] 


BLATTA  AND  DORYPHORA. 


323 


Eiern  von  Bythostrephes  steigt  das  Keimblaschen  bei  Eintritt 
der  Eireife  in  die  Hohe,  erblasst  allmahlich  und  zeigt  zugleich 
an  Eiern,  die  frisch  in  den  Brutraum  iibergetreten  sind,  den 
Beginn  der  Spindelbildung  innerJialb  des  dann  noch  scharf  her- 
vortretenden  Umrisses  des  Kleimblaschens.” 

The  interesting  researches  of  Weismann  and  Ischikawa  (49), 
Leichmann’s  (28)  recent  discovery  of  two  polar  globules  in  Asel- 
hts  and  Pereyaslawzewa’s  (40)  discovery  of  polar  globules  in 
Gam^nariis  poecilurus  prove  that  the  Crustacea^  too,  must  be 
included  under  the  general  law. 

b.  The  Laiv  of  Orientation. 

Though  considerable  attention  has  been  given  to  ookinesis  in 
Echinoderms  and  Amphibia,  no  extended  observations  of  these 
phenomena  in  the  eggs  of  Arthropods  have  been  published. 
In  view  of  this  fact,  I  have  taken  considerable  pains  to  deter¬ 
mine  the  paths  of  the  pronuclei  and  cleavage  nucleus  in  Blatta^ 
and  have  devoted  considerable  space  to  their  description. 

A  perusal  of  O.  Hertwig’s  paper  entitled,  “  Welchen  Einfluss 
iibt  die  Schwerkraft  auf  die  Theilung  thierischer  Zellen  V  (21), 
led  me  to  try  experiments  to  prove  whether  gravitation  has  any 
appreciable  effects  in  determining  the  position  of  the  embryo 
in  the  egg,  or  .whether  the  position  of  the  embryo  with  refer¬ 
ence  to  the  yolk  is  predetermined  long  before,  during  the  de¬ 
velopment  of  the  egg  in  the  follicle. 

It  has  been  shown  that  the  eggs  of  Blatta  are  carried  in  a  hori¬ 
zontal  position  after  the  first  day,  so  that  the  ventral  or  germinal 
faces  of  the  upper  row  of  eggs  are  directed  downwards,  and 
those  of  the  lower  row  upwards.  As  the  crista  is  continuous 
with  the  right  side  of  the  parent’s  body,  the  heads  of  the  em¬ 
bryos  all  point  to  the  right.  It  occurred  to  me  that  by  keeping 
capsules  in  various  positions  any  effects  of  gravitation  on  the 
development  of  the  inclosed  eggs  could  be  easily  determined. 
I  accordingly  placed  capsules  in  five  different  positions  on  a 
block  of  paraffine  provided  with  holes  and  grooves  to  keep  them 
firmly  in  place.  To  prevent  desiccation,  the  block  was  kept  in 
a  camera  humida. 

Capsules  were  kept  from  fourteen  to  twenty  days  in  the  fol¬ 
lowing  positions  :  — 


324 


WHEELER. 


[VOL.  III. 


1.  Resting  with  the  lateral  faces  perpendicular  and  crista 
uppermost. 

2.  Resting  on  the  crista  with  the  lateral  faces  perpendicular. 

3.  Resting  on  the  left  lateral  face. 

4.  Resting  perpendicularly  on  the  anterior  end. 

5.  Resting  perpendicularly  on  the  posterior  end. 

In  all  these  cases  the  eggs  developed  normally,  without  the 
slightest  indication  of  displacement  in  position  or  alteration  of 
shape  in  the  embryos ;  whether  they  were  forced  to  develop 
with  their  heads  pointing  up  or  down. 

The  development  was  slow  in  all  of  the  above  cases,  but  this 
was  not  due  to  the  unnatural  positions  of  the  capsules,  but 
rather  to  the  low  temperature  produced  by  the  evaporation  of 
the  water  under  the  bell-jar.  That  this  was  the  true  cause  was 
shown  by  capsules  kept  under  the  same  bell-jar  in  the  normal 
position,  on  the  right  side.  Their  development  was  likewise 
retarded. 

We  may  conclude  from  these  few  experiments  that  the  force 
of  gravitation  has  no  perceptible  effect  on  the  development  of 
the  eggs  of  Blatta,  but  that  these  highly  differentiated  eggs, 
utterly  unable  to  revolve  in  their  envelopes  like  the  eggs  of 
birds  and  frogs,  have  their  constituents  prearranged,  and  the 
paths  of  their  nuclei  predetermined  with  reference  to  the  parts 
of  the  embryo.  As  the  only  difference  between  the  mature 
ovarian  and  the  odthecal  egg  is  a  difference  in  shape,  we  con¬ 
clude  that  the  predetermination  is  effected  before  fecundation, 
and  even  before  the  formation  of  the  first  polar  globule. 

There  is  nothing  in  the  structure  of  the  newly  laid  egg  of 
Doryphora  to  prove  that  it  possesses  a  dorso-ventral  differentia¬ 
tion  like  the  egg  of  Blatta  ;  nor  are  my  facts  sufficient  to  war¬ 
rant  the  assertion  that  the  ventral  plate  develops  on  the  side 
opposite  the  point  at  which  the  polar  globules  arise.  Still  the 
possibility  of  such  a  condition  is  in  nowise  precluded,  and  the 
observation  of  Blochmann  and  myself  on  Blatta  probably  apply 
to  the  Hexapoda  in  general.  The  spherical  form  of  the  Crusta¬ 
cean  egg  as  opposed  to  the  oval  shape  of  the  great  majority  of 
insect  eggs,  will  be  a  great  obstacle  in  the  way  of  proving  any 
similar  conditions  in  this  lower  group. 

It  can  be  proved,  however,  that  we  have  as  true  an  anteropos¬ 
terior  differentiation  in  the  eggs  of  Doryphora  as  in  Blatta. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


525 

The  eggs  are  deposited  by  the  females  in  such  a  way  that  the 
pole  which  leaves  the  vagina  first  is  glued  to  the  surface  of  the 
leaf  by  a  semifluid  secretion,  which  at  this  point  spreads  out 
into  a  flat  disc  on  which  the  egg  rests  and  by  which  it  is 
attached.  As  the  hatching  larva  always  leaves  the  egg  head 
foremost  at  the  opposite  free  pole  of  the  egg,  and  as  there  is  no 
revolution  of  the  embryo  as  in  Hemiptera,  there  can  be  no 
doubt  as  far  as  the  cephalic  and  caudal  ends  are  concerned  that 
the  relations  of  the  embryo  to  the  parts  of  the  egg  are  the 
same  as  those  described  for  Blatta.  Moreover,  the  position  of 
the  eggs  on  the  leaf  and  the  position  of  the  embryo  in  the  egg, 
are  sufficient  evidence  that  the  eggs  are  oriented  in  the  ovaries 
of  Doryphora  with  the  cephalic  pole  directed  towards  the  head 
of  the  mother  insect. 

Thus  I  have  found  that  Blatta  completely  and  Doryphora  cer¬ 
tainly  in  part  conforms  to  the  “  loi  de  I’orientation  de  Toeuf  ” 
of  Hallez  (17),  who  found  that  the  ova  of  Hydrophihis  and 
Locusta  lie  in  the  ovaries  with  their  cephalic  ends  directed 
towards  the  head  of  the  mother  insect,  and  that  the  dorsal  and 
ventral  surfaces  of  the  egg  are  predetermined  in  the  ovaries. 
Kadyi’s  (23)  remarks  make  it  certain  that  Periplaneta  conforms 
to  the  law.  In  viviparous  Aphides  the  same  condition  obtains,  as 
may  be  gleaned  from  the  plates  of  Metchnikow  (30)  and  Will 
(52).  When  micropyles  are  developed  at  the  cephalic  pole  of 
the  egg,  they  form  a  fixed  point  which  is  of  great  assistance  in 
observing  whether  the  “law  of  orientation”  obtains  in  a  particu¬ 
lar  instance.  We  have  such  a  case  in  Corixa  as  described  by 
Metchnikow.  In  this  insect  (as  also  in  Aphis),  the  entoblastic 
growth  of  the  embryo  somewhat  obscures  the  process  ;  but  it 
can  be  readily  seen  that  when  the  ventral  plate  forms,  the  por¬ 
tion  of  it  which  will  subsequently  grow  out  into  the  procephalic 
lobes  is  situated  at  the  micropylar  pole,  which  is  anteriorly  di¬ 
rected  while  in  the  body  of  the  mother  insect.  The  growing  into 
the  yolk  of  the  embryo  tail  first  brings  the  head  to  the  opposite 
end  of  the  egg,  but  during  revolution  the  embryo  regains  the 
position  which  it  held  before  the  formation  of  the  amnion  and 
serosa  (see  Metchnikow  (30),  Plates  XXVT.  and  XXVII. 
Figs.  6,  II,  20,  25,  27). 


326 


WHEELER. 


[VOL.  III. 


The  Formation  of  the  Germ  Layers  and  Embryonic 

Envelopes. 

DorypJio7'a. 

As  my  observations  on  the  formation  of  the  germ  layers  in  the 
potato-beetle  are  both  more  copious  and  more  satisfactory  than 
in  Blatta,  and  as  Doryphora  probably  represents  more  nearly 
the  typical  process  of  germ  layer  formation,  I  will  begin  my 
remarks  on  this  subject  with  DorypJiora,  and  append  what  I 
have  to  say  on  Blatta. 

The  first  change  visible  in  the  blastoderm  from  the  surface  is 
the  appearance  of  a  pair  of  folds  which  arise  on  the  middle  of 
what  is  to  be  the  ventral  surface  of  the  egg  (Fig.  66).  The  two 
folds  may  best  be  described  as  resembling  a  pair  of  but  slightly 
bent  parentheses  close  together.  In  cross-sections  these  folds 
are  scarcely  perceptible,  and  the  difference  in  thickness  between 
the  ventral  and  dorsal  blastoderm  is  very  slight. 

Soon  the  folds  became  more  decided  (Figs.  67  and  68),  their 
anterior  ends  are  continued  around  and  meet  so  as  to  inclose 
a  spade-shaped  space,  while  their  posterior  ends  diverge  and  are 
continued  in  the  opposite  direction  to  the  caudal  end  of  the  egg, 
where  the  depression  inclosed  between  the  two  folds  turns 
inward  and  ends  abruptly.  The  depression  inclosed  by  the 
fold  is  to  become  the  groove-shaped  gastrula.  The  nuclei  of 
the  blastoderm  now  present  a  very  different  appearance  from 
that  represented  in  Fig.  66.  The  fold-surrounded  depression 
at  the  anterior  end  (Fig.  67  d),  lying  where  the  stomodaeum 
is  subsequently  invaginated,  is  apparently  a  differentiated  por¬ 
tion  of  the  gastrula,  to  judge  from  its  peculiar  shape.  In 
lateral  view  (Fig.  68  d)  this  portion  of  the  blastoderm  is  seen 
to  be  concave,  the  nuclei  are  closely  aggregated,  whereas  an¬ 
teriorly  and  posteriorly  they  are  much  scattered  and  have 
increased  in  size.  The  true  distribution  of  the  cells,  as  shown 
by  their  nuclei,  is  best  seen  in  the  lateral  view  of  the  same 
egg  (Fig.  68).  The  aggregated  mass  of  cells,  or  the  ventral 
plate  as  it  may  now  be  called,  is  clearly  marked  off  from  the 
serosa  or  remaining  blastoderm,  which  .  is  recognized  by  its 
larger  and  more  scattered  nuclei.  The  ventral  plate  is  seen 
to  be  constricted  toward  the  middle  of  the  egg  to  form  two 


No.  2.] 


BLATTA  AND  DORYPHORA. 


327 


lobes  ;  the  anterior  broader  and  shorter  is  the  procephalic,  and 
the  posterior  and  longer  the  abdomino-thoracic  lobe.  In  both 
views  of  the  egg  (Figs.  67  and  68)  a  number  of  lines  are  seen 
crossing  the  ventral  plate  at  right  angles  to  the  longer  diam¬ 
eter.  These  lines  make  the  plate  appear  segmented,  and  at 
first  reminded  me  of  Kowalevsky’s  figure  (Plate  VIII.,  Fig.  2), 
which  represents  an  embryo  HydropJiiliis  in  exactly  the  same 
stage  as  that  which  I  have  figured.  More  careful  examination, 
however,  convinced  me  that  the  lines  were  not  due  to  segmen¬ 
tation,  nor,  in  fact,  depressed  at  all,  but  were  the  wrinkles  into 
which  the  ventral  plate  was  thrown,  probably  by  a  contraction 
away  from  the  anterior  pole. 

The  simple  method  by  which  the  ventral  plate  is  formed  is 
easily  seen  in  a  median  cross-section  of  an  egg  in  the  stage  fig¬ 
ured  in  Fig.  67  (Fig.  64).  The  small  projection  r  is  the  ridge 
which  separates  the  thick  and  sinking  gastrular  portion  of  the 
blastoderm  {g)  from  the  remainder  of  the  layer.  As  we  have 
seen,  all  the  cells  of  the  completed  blastoderm  are  columnar.  The 
thickened  ventral  plate  is  formed  merely  by  the  cells  on  the 
ventral  surface  of  the  egg  lengthening  in  a  radial  direction,  and 
those  of  the  dorsum  (P'ig.  647)  changing  their  shape  so  as  to 
have  their  long  axes  tangentially  directed. 

With  further  development  the  gastrular  groove  deepens  and 
the  ridges  come  close  together  in  the  median  line  (Fig.  70). 
The  oral  end  of  the  gastrula  {ci)  is  oval,  and  is  no  longer  marked 
off  anteriorly  by  the  ridge  seen  in  Fig.  68.  On  each  side  of  the 
oral  widening  is  seen  a  small  fold  {ani)  between  which  and 
the  gastrula  the  ventral  plate  cells  are  much  thickened.  This 
fold  is  the  beginning  of  that  portion  of  the  amnion  which  sub-. 
sequently  envelopes  the  head,  and  the  thickenings  are  probably 
the  first  traces  of  the  brain.  The  egg  (P'ig.  70)  viewed  from 
the  posterior  end  discloses  some  interesting  facts  (Fig.  69), 
The  ventral  plate  has  become  pushed  in  at  this  point,  and  the 
thickened  lip  thus  formed  grows  forward  over  it.  This  lip  is 
the  beginning  of  the  caudal  fold  of  the  amnion  and  serosa.  The 
gastrular  invagination  is  considerably  deeper  at  its  posterior 
than  at  its  oral  end. 

The  caudal  fold  of  the  amnion  and  serosa  grows  much  more 
rapidly  than  the  cephalic  folds.  The  tail  end  of  the  ventral 
plate  advances  dorsally  till  it  is  one-third  the  length  of  the  egg 


328 


WHEELER. 


[VOL.  III. 


from  the  anterior  pole.  It  pushes  its  way  through  the  yolk  in 
such  a  manner  as  not  to  bring  the  amnion  or  serosa  in  close 
apposition,  but  to  inclose  a  greater  or  less  amount  of  yolk 
between  the  two  membranes.  This  dorsal  growth  of  the  caudal 
end  shows  considerable  variation,  however.  In  some  eggs  the 
end  of  the  tail  comes  to  lie  at  such  a  distance  from  the  dorsal 
surface  that  it  is  almost  in  the  centre  of  the  yolk.  In  other 
instances  the  amnion  and  serosa  become  closely  apposed  and 
extrude  the  yolk  from  between  them  soon  after  the  two  enve¬ 
lopes  have  closed  over  the  mouth,  and  the  embryo  has  attained 
its  maximum  length. 

In  Fig.  71  I  have  represented  the  much  curved  embryo  in  this 
stage  straightened  out.  The  gastrular  invagination  has  not  yet 
closed,  but  it  is  much  narrowed  and  more  lengthened  on  account 
of  the  greater  growth  in  length  of  the  embryo.  The  oral  end 
is  still  wider  than  the  remaining  portion.  At  the  anal  end  (Fig. 
71  x)  the  groove  seems  to  bifurcate.  At  the  point  x  the  procto- 
deeum  is  subsequently  invaginated.  Usually  in  embryos  as  old 
as  that  represented  the  anterior  half  of  the  gastrula  has  closed 
completely. 

In  the  embryo  figured  the  cephalic,  maxillary,  thoracic,  and 
abdominal  portions  are  already  marked  out.  The  first  begin¬ 
nings  of  the  three  pairs  of  legs  are  apparent  in  the  undulating 
edge  of  the  thoracic  portion  of  the  ventral  plate 

A  median  cross-section  of  the  egg  in  the  stage  just  described 
cuts  the  embryo  at  right  angles  to  its  long  diameter  in  two 
places  (Fig.  65).  At  the  posterior  end  the  thick  amnion  {am) 
is  separated  from  the  serosa  by  a  layer  of  yolk  {y),  which  will 
shortly  be  pushed  out  from  either  side  to  join  the  great  central 
yolk  mass.  In  this  half  of  the  section  which  is  near  the  caudal 
end  the  gastrula  is  still  open,  though  its  cells  have  ceased  to  be 
columnar,  and  are  dividing  rapidly  to  form  the  thick  lump  of 
cells  so  very  conspicuous  at  this  point  in  slightly  later  stages. 
In  the  anterior  portion  of  the  egg  the  gastrula  has  closed,  and 
the  ectoderm  and  mesoderm  are  clearly  separated.  The  tubular 
walls  of  the  gastrula  have  broken  down  to  form  an  irregular 
mass  of  polygonal  cells  which  lie  in  the  median  line  closely 
applied  to  the  outwardly  convex  ectoderm.  Though  the  clos¬ 
ing  of  the  gastrula  progresses  from  before  backwards,  the 
closure  of  the  oral  portion  is  retarded  and  is  marked  by  a 


No.  2.] 


BLATTA  AND  DORYPHORA. 


329 


narrow  oval  pit  which  is  visible  till  the  mouth  begins  to  in- 
vaginate  (Fig.  73  d). 

While  the  amnion  and  serosa  are  closing  over  the  oral  portion 
of  the  embryo  the  yolk  begins  to  segment.  The  first  traces  of 
segmentation  are  visible  on  the  dorsal  surface  between  the  end 
of  the  tail  and  the  procephalic  lobes.  There  the  surface  of  the 
yolk  assumes  a  scalloped  appearance,  and  radially  directed  lines 
soon  mark  the  divisions  between  the  yolk  balls.  The  segmen¬ 
tation  progresses  thence  in  a  ventral  direction,  both  from  either 
side  of  the  dorsum  and  directly  inwards,  so  that  finally  the  whole 
yolk  is  reduced  to  rounded  masses,  each  of  which  contains  from 
one  to  three  vitellophags.  Each  yolk  segment  is  properly  a  cell 
with  its  protoplasm  radiating  in  all  directions  as  a  delicate 
reticulum  which  holds  in  its  meshes  the  unequal-sized  yolk 
spheres.  By  the  time  the  amnion  and  serosa  have  completely 
formed,  all  the  yolk  has  been  converted  into  distinct  subspher- 
ical  segments,  except  the  portions  immediately  under  the  anal 
and  oral  ends  of  the  gastrula.  Here  the  segmentation  remains 
for  a  short  time  indistinct  till  the  entoderm  is  established  at 
these  points. 

In  the  embryo  with  completely  closed  envelopes  (Fig.  73, 
PL  V.)  the  procephalic  lobes  {pci)  have  grown  in  size,  and 
when  attached  to  the  egg  clasp  its  upper  pole.  The  three  pairs 
of  thoracic  limbs  are  distinctly  formed,  while  of  the  cephalic 
appendages  only  the  2d  maxillae  {mx  2)  are  beginning  to  appear. 
The  base  of  the  attenuated  ribbon-shaped  abdomen  shows  traces 
of  commencing  segmentation.  Posteriorly  it  suddenly  widens 
out  into  a  flat,  transversely  oval  body,  which  I  shall  call  the  caudal 
plate.  The  gastrular  invagination  is  closed  except  at  its  anal 
end  {x  73),  and  the  mouth  will  soon  form  at  the  shallow  oval 
depression  which  marks  the  anterior  end  of  the  groove. 

Fig.  82  represents  a  longitudinal  section  through  an  embryo 
a  very  little  older  than  the  one  just  described  (Fig.  73). 

The  serosa  {sr)  now  covers  the  entire  egg,  and  is  separated 
from  the  embryo  proper  and  the  amnion  {ani).  Both  mem¬ 
branes  present  the  same  appearance  in  section,  being  nodulated 
with  nuclei.  The  anterior  end  of  the  embryo  lies  on  the  anterior 
end  of  the  yolk  in  such  a  position  that  its  mouth  {st)  is  almost 
exactly  at  the  pole ;  the  tail  ends  somewhat  anterior  to  the  mid¬ 
dle  of  the  dorsal  surface  W-  The  ectoderm,  which  at  its  cephalic 


330 


WHEELER. 


[VoL.  III. 


and  caudal  ends  {p  and  p)  passes  imperceptibly  into  the  much 
thinner  amnion,  is  considerably  thickened  and  has  its  crowded 
nuclei  in  several  rows,  though  but  one  row  of  cells  is  present. 
The  depressions  which  mark  off  the  incipient  appendages  are 
deep  and  narrow.  The  mesoderm  {msd)  is  spread  out  under 
the  whole  of  the  ectoderm  and  has  begun  to  thicken  under 
each  somite  preparatory  to  segmentation.  It  is  very  noticeably 
thickened  in  two  places  :  under  the  stomodaeal  depression  (//^) 
and  under  the  caudal  plate  (//^),  where  it  forms  a  large  mass  of 
cells  projecting  into  the  as  yet  unsegmented  yolk  just  beneath  it. 
These  two  masses  of  cells  are  the  mdependent  sources  of  the  ento¬ 
derm,  which  grows  backwards  as  two  strmgs  f'om  the  anterior 
mass  {pi  ^),  a}id  forward  as  two  strings  from  the  posterior  mass 
{pl‘^).  As  we  shall  see  further  on,  these  two  strings  nnite  near 
the  middle  of  the  body  and  then  begin  to  groiv  at  their  lateral 
edges  till  the  mesenteron  thus  formed  incloses  the  yolk. 

The  points  from  which  the  chords  grow  are  plainly  seen  in 
the  figure  {ent^  and  ent‘^).  Under  both  points  of  proliferation 
there  are  a  number  of  nuclei  which  at  first  sight  under  a  low 
power  seem  to  be  dividing  karyokinetically.  The  chromatin  is 
all  aggregated  in  one  or  two  dense  masses  in  the  hyaline  karyo- 
chylema,  and  thus  resembles  the  similar  aggregations  seen  in 
kinetic  nuclei.  These  nuclei,  however,  are  not  dividing,  but 
undergoing  decomposition,  as  we  shall  see  when  we  come  to 
examine  a  more  highly  magnified  section  through  the  caudal 
plate. 

Before  leaving  Fig.  82  I  would  call  attention  to  the  three  cells 
at  c  which  are  on  the  surface  of  the  embryo  in  the  amniotic 
cavity.  They  are  very  large  and  clear,  and  the  more  anterior 
is  apparently  creeping  in  the  manner  of  an  Amoeba  along  the 
surface  of  the  abdominal  ectoderm.  These  cells,  the  ultimate 
fate  of  which  I  have  been  unable  to  determine,  probably  escape 
from  the  anal  orifice  of  the  gastrula  before  it  closes.  I  have 
in  several  cases  seen  such  cells  issuing  from  or  still  in  connec¬ 
tion  with  the  infolded  pocket  of  ectoderm,  which  is  called  meso¬ 
derm  as  soon  as  the  outer  layer  has  closed  over  it  (Fig.  87  c). 
These  peculiar  cells  may  be  the  homologues  of  the  ‘  Polzellen  ’ 
long  ago  observed  in  certain  Diptera. 

A  much  clearer  understanding  of  the  method  of  formation  of 
the  entoderm  may  be  obtained  from  Figs.  87  and  88,  both  rep- 


No.  2.] 


BLATTA  AND  DORYPHORA. 


33r 


resenting  cross-sections  through  the  middle  of  the  caudal  plate. 
In  Fig.  87,  from  a  younger  embryo,  the  gastrula  has  not  yet 
closed.  Its  walls  are  seen  to  be  much  thickened,  and  the  kary- 
okinetic  figures  show  that  its  component  cells  are  still  prolifer¬ 
ating.  At  the  lower  surface  of  the  bag-shaped  mass  the  cells 
are  somewhat  less  compact  and  form  a  layer  {ent)  which  at  some 
points  is  separated  from  the  superjacent  cell-mass.  This  mass 
will  give  rise  to  the  entoderm,  and  that  above  it  to  the  meso¬ 
derm,  as  soon  as  the  orifice  .r  is  definitely  closed.  We  thus  have  a 
mass  of  cells  in  which  all  three  germ  layers  hlend^  and  to  no  part 
of  which  ca7i  %e  assigned  the  7iame  of  a  germ  layer.  Not  till  the 
groove  is  closed  have  we  mesoderm,  and  not  till  the  lower  cells  of 
the  7nass  have  beco77ie  clea7'ly  differentiated fro7n  those  above  them 
ca7i  we  speak  of  e7itoder7n. 

The  section  Fig.  88  is  from  an  embryo  in  which  three  germ 
layers  are  definitely  formed,  shortly  after  the  closing  of  the  gas¬ 
trula.  A  depression  (x)  marks  the  point  where  the  proctodaeal 
invagination  is  to  occur.  The  polygonal  mesoderm  cells  are 
spread  out  in  a  mass  {77isd),  which  is  separated  by  a  more  or  less 
distinct  line  {/)  from  the  entoderm  beneath  {e7it).  The  differ¬ 
ences  between  the  cells  of  the  last  layer  and  the  superjacent 
mesoderm  are  difficult  to  represent.  Their  nuclei  are  somewhat 
larger  and  clearer.  They  gradually  merge  into  the  mesoderm 
cells,  the  boundary  being  exceptionally  clear  in  the  section  fig¬ 
ured.  H eider  (19)  says  of  these  same  entoderm  cells  in  Hydro- 
philus  that  they  are  more  “succulent”  than  the  mesodermic 
elements.  This  adjective  conveys  the  idea  more  clearly  than 
paragraphs  of  description. 

The  peculiar  nuclei  which  under  a  low  power  seemed  to  be 
dividing  are  now  seen  to  be  in  a  process  of  dissolution.  They 
origiTiate  m  the  e7itoder7nic  7nass  a7td pass  mto  the  adjace7it  yolk, 
wlio'e  they  disappear,  sections  through  slightly  later  stages 
showing  no  traces  of  them.  From  what  I  have  seen  I  believe 
these  nuclei  to  pass  through  the  following,  stages,  examples  of 
all  of  which  may  be  found  in  a  single  embryo.  The  karyochy- 
lema  becomes  vacuolated,  probably  with  substances  absorbed 
from  without,  to  judge  from  the  large  size  of  some  of  these 
nuclei  (Fig.  88  v),  while  the  chromatin  ceases  to  present  the 
threadlike  coil  and  becomes  compacted  into  irregular  masses  be¬ 
tween  the  vacuoles.  Finally,  the  vacuoles  fuse  and  the  masses  of 


332 


WHEELER. 


[VoL.  iir. 


chromatin,  formerly  numerous,  agglomerate  to  form  one  or  two 
large  irregular  masses  which  usually  apply  themselves  to  the 
wall  of  the  clearly  vesicular  nucleus  (Fig.  88  t).  The  wall  of 
the  nucleus  then  ceases  to  be  evenly  spherical,  and  becomes 
irregular  apparently  because  the  karyochylema  is  escaping 
through  a  rent  (Fig.  88  6).  In  the  last  stages  seen  the  masses 
of  chromatin  lie  between  the  yolk  bodies,  all  the  other  portions 
of  the  nucleus  having  disappeared.  They  still  take  the  char¬ 
acteristic  deep  red  stain,  but  finally  become  comminuted  and 
disappear  in  the  intervitelline  protoplasm. 

The  dissolution  of  these  nuclei  and  their  migration  into  the 
yolk  is  brought  to  a  close  soon  after  the  entodermic  mass  begins 
to  grow  forward. 

The  oral  mass  of  proliferating  cells  is  essentially  the  same  as 
the  caudal  mass  just  described ;  but  being  smaller,  I  have  not 
seen  fit  to  represent  it  in  the  plates  by  enlarged  figures.  Some 
of  the  entoderm  nuclei  degenerate  in  exactly  the  same  manner 
as  those  described  in  the  caudal  thickening,  but  the  whole  mass 
of  cells  being  smaller,  the  number  of  these  evanescent  nuclei  is 
much  less. 

I  am  at  a  loss  to  assign  a  meaning  to  this  migration  of  de¬ 
generating  entoderm  nuclei  into  the  yolk  unless  it  be  supposed 
that  originally  all  the  nuclei  of  the  egg  went  to  the  surface  and 
that  a  portion  of  the  entoderm  passed  into  the  yolk  to  form  vitel- 
lophags  while  another  portion  proliferated  forward  in  compact 
sheets  to  form  the  walls  of  the  mesenteron.  Later,  when  the 
ontogeny  was  abbreviated  in  the  blastoderm  stage  by  cells  being 
left  in  the  yolk,  this  migration  of  entoderm  cells  became  unnec¬ 
essary,  as  the  yolk,  which  is  already  segmented,  is  copiously  sup¬ 
plied  with  vitellophags.  The  lack  of  distinct  yolk  segmentation 
just  beneath  the  two  proliferating  points  may  lend  some  proba¬ 
bility  to  this  view.  I  am  aware  that  my  explanation  halts,  but 
it  will  have  to  stand,  for  the  want  of  a  better  one,  till  more  facts 
are  forthcoming  on  these  degenerating  nuclei  in  other  forms. 

In  examining  the  literature  the  only  observation  which  I  can 
find  similar  to  the  one  just  recorded  is  in  Hatschek’s  paper  on 
Bonibyx  cJirysorrJioea  (i8).  He  observed  a  mass  of  nuclei,  which 
in  his  figures  have  all  the  appearance  of  undergoing  degenera¬ 
tion,  just  anterior  to  the  large  mass  of  entoderm  cells  attached 
to  the  oral  ectoderm.  This  mass  of  nuclei,  designated  by  him 
as  a  “gland,”  soon  disappears  in  the  yolk. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


333 


Blatta. 

My  observations  on  the  formation  of  the  germ  layers  in  Blatta 
are  less  satisfactory  than  those  on  the  same  process  in  DorypJiora^ 
because  the  eggs  of  the  former  are  difficult  to  section  and  have 
small,  indistinct  cells  in  the  later  stages.  I  have,  however, 
given  much  attention  to  the  subject,  sufficient,  I  believe,  to  be 
able  to  assert  that  the  method  of  germ  layer  formation  departs 
from  the  type  observed  in  HydropJiiliis  and  DorypJiora. 

As  soon  as  the  blastoderm  is  completed  by  the  rapid  prolifer¬ 
ation  of  the  blastema  cells,  the  whole  layer  of  protoplasm 
with  its  embedded  nuclei  contracts  from  the  lateral  faces 
towards  the  front  of  the  egg.  The  blastoderm  thus  becomes 
exceedingly  thin  on  the  lateral  and  dorsal  surfaces  of  the  egg, 
and  the  nuclei  of  these  surfaces  become  much  scattered  and 
flattened,  while  the  protoplasm  is  thickened  on  the  whole  ven¬ 
tral  face,  where  the  nuclei  are  crowded  together  and  have  again 
become  spherical.  A  slight  further  contraction  away  from  the 
cephalic  and  caudal  ends  towards  the  centre  of  the  ventral  face 
shortens  this  mass  of  thickened  cells  into  the  ventral  plate. 

While  the  blastoderm  is  thickening,  nuclei  are  being  given 
off  centripetally  to  form  the  yolk  cells.  A  few  of  these  nuclei 
go  deep  into  the  yolk,  but  the  great  majority  remain  at  or  very 
near  the  surface.  They  are  not  given  off  in  a  continuous  sheet, 
nor  are  they  produced  from  the  blastoderm  by  any  invagination. 
They  are  simply  nuclei  which  have  been  sent  into  the  yolk  from 
different  and  often  widely  separated  points  of  the  contracting 
blastoderm.  The  few  nuclei  which  descend  into  the  yolk 
remain  for  a  long  time  small  and  indifferent.  Sometimes  the 
number  of  these  nuclei  is  very  limited  so  that  they  occur  in 
only  a  few  of  a  great  number  of  complete  longitudinal  sections 
passed  through  an  egg. 

The  nuclei  of  the  surface  yolk  undergo  considerable  differen¬ 
tiation,  and  are  soon  easily  distinguished  from  the  superjacent 
blastoderm.  They  surround  themselves  with  stellate  cytoplasm, 
retain  their  spherical  or  spheroidal  shape,  and  often  present  one 
or  more  large  nucleoli  (Figs.  29  and  32  v).  Their  function  for 
many  days  is  the  conversion  of  the  yolk  into  soluble  compounds 
to  be  absorbed  by  the  rapidly  dividing  cells  of  the  embryo. 
During  this  process  they  grow  rapidly,  and  soon  become  the 


3’34 


WHEELER. 


[VoL.  iir. 


largest  cells  with  the  largest  nuclei  in  the  egg  (Fig.  30  v).  The 
long  pseudopodial  continuations  of  the  finely  and  evenly  gran¬ 
ular  cytoplasm  can  often  be  traced  for  a  considerable  distance 
between  the  yolk  polyhedra.  The  yolk  cells  are  never  seen 
in  process  of  division,  and  as  their  number  in  eggs  of  widely 
different  stages  is  approximately  constant,  I  conclude  that  they 
rarely  or  never  divide. 

The  thickening  process  which  formed  the  ventral  plate  still 
continues  in  a  spot  about  one-fourth  the  length  of  the  egg  from 
the  caudal  end,  and  gives  rise  to  a  rounded  mass  of  cells  which 
are  much  thicker  than  the  surrounding  portion  of  the  ventral 
plate  though  but  slightly  raised  above  the  general  surface  (Fig. 
41).  A  faint  depression  appears  in  the  centre  of  the  rounded 
mass  {bp).  While  this  thickening  is  forming,  the  nuclei  of  the 
ventral  plate  are  also  proliferating  very  rapidly  at  two  points  on 
the  ventrolateral  edges  about  one-fourth  the  length  of  the  egg 
from  the  cephalic  end  (Fig.  41  pci).  These  cells  also  increase 
in  depth,  but  do  not  rise  above  the  general  surface  of  the  egg. 
The  two  patches  of  thickened  cells  are  the  precursors  of  the 
procephalic  lobes.  The  deceptive  appearance  of  a  groove  is 
presented  by  the  keel  {cfi),  which  runs  the  whole  length  of  the 
ventral  face,  but  soon  disappears  as  it  is  absorbed  by  the  young 
embryo. 

The  formation  of  the  mesoderm  can  be  traced  in  eggs  sec¬ 
tioned  during  or  shortly  before  the  stage  figured  in  Fig.  41. 
Figure  38  is  a  longitudinal  section  through  the  posterior  portion 
of  the  egg  (Fig.  41)  through  the  middle  of  the  thickened  mass 
of  cells  and  the  depression  bp.  In  this  egg  the  mesoderm  has 
been  forming  for  some  time.  The  ventral-plate  cells  are  col¬ 
umnar  at  bp,  and  their  nuclei  are  elongated  in  just  the  opposite 
direction  to  their  former  longest  axis,  which  is  still  the  longest 
axis  of  the  serosa  nuclei  at  sr.  The  depth  of  the  ventral-plate 
cells  gradually  decreases  anteriorly.  The  stellate  yolk  cells  are 
scattered  at  various  distances  from  one  another  under  the  ven¬ 
tral  plate.  The  mesoderm  {msd),  as  is  clearly  seen  from  the 
section,  arises  partly  from  proliferation  of  the  ventral-plate  cells 
under  bp  and  passes  forward  as  a  single  but  incomplete  layer 
of  cells.  Towards  the  head  this  layer  splits,  and  each  of  the 
two  bands  thus  formed  continues  forwards  under  one  of  the 
procephalic  thickenings.  This  is  seen  in  Fig.  31  from  a  sec- 


No.  2.] 


BLATTA  AND  DORYPHORA. 


335 


tion  through  the  point  pci  of  the  egg  represented  in  Fig.  41.  ecd 
is  the  thickened  ectoderm,  msd  the  mesoderm,  which  is  absent 
in  the  middle  of  the  ventral  face  at  c. 

That  the  mesoderm  grows  forward  from  the  rounded  caudal 
thickening  may  be  proved  by  sections  through  a  number  of 
eggs  taken  from  capsules  six  and  one-half  to  eight  days  old. 
The  eggs  will  be  found  in  various  stages  more  or  less  close  to 
that  given  in  Fig.  41.  In  some  a  very  short  row  of  mesoderm 
cells  is  found  just  in  front  of  the  thickening ;  in  others  the  row 
will  be  longer  as  it  has  advanced  further  to  the  cephalic  end. 
Only  part  of  the  mesoderm  is  formed  at  the  thickening.  As 
can  be  concluded  from  the  even  arrangement  of  the  two  layers 
anterior  to  bp  in  Fig.  38,  each  ectoderm  cell  has  a  mesoderm 
cell  beneath  it,  showing  that  the  mesoderm  is  derived  from  the 
ectoderm  by  centripetal  division.  The  impulse  to  this  division, 
however,  seems  to  originate  in  the  incomplete  invagination  at 
bp  and  to  travel  towards  the  head  of  the  embryo. 

After  the  mesoderm  is  formed,  the  depression  bp  disappears, 
and  the  amnion  and  serosa  begin  to  develop.  They  rise  as  a 
crescentic  fold  from  the  rounded  posterior  edge  of  the  area  of 
proliferation  (Fig.  42  as).  The  cells  of  the  procephalic  lobes 
become  more  prominent,  and  while  the  caudal  fold  of  the 
amnion  and  serosa  is  growing  in  length  and  continuing  up  the 
edge  of  the  ventral  plate,  a  fold  also  arises  from  the  outer  edge 
of  each  procephalic  area  and  bends  inward.  This  stage  in  the 
development  of  these  membranes  is  seen  in  Fig.  43. 

The  embryo  is  now  slipper-shaped,  the  toe  of  the  slipper 
being  the  caudal  and  the  heel  the  cephalic  end.  The  growth  of 
the  membranes  continues,  the  toe  of  the  slipper  completing 
itself  more  rapidly  than  the  heel.  Soon  the  two  procephalic 
folds  are  connected  around  the  anterior  tip  of  the  ventral  plate, 
which  is  undergoing  a  change  in  outline.  Figure  44  shows  the 
amnion  and  serosa  almost  closed.  Over  the  spot  where  the 
stomodaeal  invagination  will  soon  appear,  there  is  still  a  small, 
slit-shaped  opening  in  the  membranes,  but  this  soon  closes ; 
not,  however,  till  after  the  appendages,  both  cephalic  and  tho¬ 
racic,  have  begun  to  appear. 

The  structure  and  formation  of  the  amnion  and  serosa,  as  well 
as  their  relations  to  each  other,  can  be  made  out  from  the  sec¬ 
tions  (Figs.  29,  30,  and  32).  Figure  29  is  a  transverse  section 


336 


WHEELER. 


[VOL.  III. 


through  the  point  c  of  Fig.  43,  a  region  at  which  the  two  folds 
have  not  yet  arisen.  The  ectoderm  {ecd)  is  very  thick,  espe¬ 
cially  in  the  median  ventral  line.  The  mesoderm  i^nsd)  is 
incompletely  separated  from  the  ectoderm,  and  is  seen  only  in 
the  median  portion  of  the  section.  The  yolk  cells  are  large  and 
distinct.  Their  chromatin  forms  large  nucleoli.  The  yolk  under 
the  embryo  is  becoming  much  vacuolated. 

The  section  (Fig.  32)  passes  through  a  point  a  little  in  front  of 
as  in  Fig.  43.  This  embryo  was  older  than  the  one  represented 
in  Fig.  42  ;  hence  the  yolk  cell  {v)  is  much  larger  and  the  meso¬ 
derm  more  distinctly  separated  from  the  ectoderm.  The  infold¬ 
ing  of  the  ectoderm  to  form  the  amnion  and  serosa  is  seen  on 
either  side.  The  amnion  {avi)  is  thicker  than  the  serosa  isi). 
Its  nuclei  are  small,  close  together,  and  spherical,  while  the 
nuclei  of  the  serosa  are  large,  flat,  and  scattered.  Figure  30  is  a 
section  through  a  point  near  (Fig.  43),  of  a  slightly  older  em¬ 
bryo.  The  amnion  and  serosa  are  completed  and  in  close  con¬ 
tact  with  each  other,  so  that  the  yolk  cannot  pass  in  between 
them.  The  difference  between  the  nuclei  of  the  amnion  and 
serosa  is  very  pronounced.  The  mesoderm  {insd),  which  is 
several  cells  deep,  and  is  now  distinctly  separated  from  the 
ectoderm,  has  become  an  independent  layer.  Its  nuclei  are 
more  spherical  than  the  ectoderm  nuclei,  many  of  which  are 
considerably  elongated  and  flattened. 

The  entoderm  is  formed  very  late  in  Blatta  (about  the  15th 
or  1 6th  day),  and  in  cross-sections  of  embryos  of  that  age  it 
may  be  seen  as  a  thin  layer  of  cells  on  either  side  of  the  ventral 
yolk  closely  applied  to  the  inner  face  of  the  splanchnic  meso¬ 
derm  (Fig.  54  eiii).  Cholodkovsky  (10)  claims  that  these  bands 
of  entoderm  are  derived  from  the  splanchnic  mesoderm  by  de¬ 
lamination,  but  an  examination  of  a  number  of  sections  has 
convinced  me  that  it  is  next  to  impossible  to  come  to  any  defi¬ 
nite  conclusion  as  to  the  mode  of  origin  of  the  entoderm  in 
Blatta.  By  the  time  it  has  begun  to  form  the  cells  have  become 
very  minute,  and  different  tissues  of  very  different  origins  have 
become  closely  united,  so  that  the  proximity  of  splanchnic  meso¬ 
derm  and  entoderm  is  no  proof  of  the  derivation  of  the  latter 
from  the  former.  I  have  been  able  to  satisfy  myself,  however, 
that  the  entoderm  appears  in  two  thin  layers,  one  on  each  side 
of  the  median  ventral  yolk,  and  that  these  two  layers  converge 


No.  2.] 


BLATTA  AND  DORYPHORA. 


'y  n  y 
00/ 

anteriorly  and  posteriorly,  and  become  united  to  the  inner  ends 
of  the  stomodaeal  and  proctodaeal  pockets,  in  a  manner  which 
differs  in  no  respect  from  what  is  observed  in  Doryphora  after 
the  entoderm  has  become  definitely  established.  I  can  there¬ 
fore  see  no  reason  for  adopting  Cholodkovsky’s  view  as  proving 
that  the  entoderm  arises  in  a  manner  differing  from  that  de¬ 
scribed  above  for  Doryphora.  As  far  as  Blatta  is  concerned 
the  question  of  the  formation  of  the  inmost  germ  layer  must 
still  be  regarded  as  an  open  one. 

The  embryo  is  hammer-shaped  when  the  amnion  and  serosa 
have  closed.  The  cephalic  lobes  extend  around  on  to  the  lateral 
surfaces  of  the  yolk,  and  each  soon  becomes  divided  into  two 
lobes,  a  larger  anterior  and  a  smaller  posterior  one  (Fig.  44). 
The  posterior  lobe  is  the  commencement  of  the  antenna  {at). 
In  many  embryos  one,  or,  more  rarely,  both  antennary  lobes 
are  temporarily  bilobed  (Fig.  44).  This  may  be  a  slight  rever¬ 
sion,  tending  to  show  that  the  antennary  lobes  originally  gave 
rise  to  two  pairs  of  appendages  which  were  perhaps  homologous 
with  the  two  pairs  of  antennae  in  the  Crustacea.  The  mandi¬ 
bles  and  first  and  second  maxillae  are  just  visible  as  faint  out¬ 
growths  of  the  ectoderm,  while  the  more  pronounced  three  pairs 
of  thoracic  legs  have  produced  a  widening  of  the  embryo  in  the 
middle  of  its  length  (Fig.  44/). 

The  time  required  for  the  development  described  in  the  pre¬ 
ceding  paragraphs  is  about  four  days.  The  blastoderm  begins 
to  contract  and  thicken  on  the  seventh  day  from  the  commence¬ 
ment  of  development,  and  by  the  end  of  this  day  the  area  of 
proliferation  is  formed.  Early  on  the  eighth  day  the  amnion 
and  serosa  begin  to  appear.  They  develop  rapidly,  so  that  the 
embryo  is  in  the  slipper  stage  by  the  close  of  the  eighth  day. 
The  amnion  and  serosa  are  almost  or  quite  closed  by  the  end 
of  the  ninth  day.  During  the  tenth  day  the  mesoderm  is  seg¬ 
mented,  and  the  cephalic  and  thoracic  appendages  begin  to 
appear.  The  embryonal  envelopes  are  completely  formed  by 
the  end  of  the  tenth  day,  and  the  embryo  has  assumed  the 
shape  of  a  hammer  or  mallet. 


338 


WHEELER. 


[VOL.  III. 


General  Remarks. 
a.  Germ  layer's. 

Our  knowledge  of  the  formation  of  the  germ  layers  in  the 
Orthoptera  is  still  less  satisfactory  than  in  other  orders  of 
insects,  as  in  most  of  the  species  studied  even  the  formation 
of  the  mesoderm  has  not  been  clearly  determined. 

Korotneff  (25)  failed  to  find  the  typical  groove-shaped  gas- 
trula  in  Gryllotalpa,  and  Ayers  (i)  had  no  better  success  with 
CEcantJms.  On  the  other  hand,  Bruce  (7)  has  observed  the 
typical  process  of  mesoderm  formation  in  Mantis,  and  Graber  (i  5) 
in  Mantis  and  Stenobothrus.  Thus  only  representatives  of  the 
Orthopteran  families  Mantidce  and  Acridiidce  are  known  to  form 
their  mesoderm  in  the  typical  manner. 

Nusbaum  (33)  concludes,  on  apparently  very  little  evidence, 
that  the  mesoderm  of  Blatta  is  formed  in  the  same  manner 
as  in  Mnsca,  and  he  gives  a  figure  of  a  cross-section  with  an 
immense  invagination. 

Cholodkovsky  (10),  in  his  account  of  the  entoderm  formation 
in  Blatta,  says  that  the  gastrula  which  gives  rise  to  the  inner 
layer  is  very  easily  observed. 

As  I  spent  much  time  on  a  full  series  of  stages  between 
the  formation  of  the  ventral  plate  and  the  appearance  of  the 
appendages,  without  being  able  to  find  a  trace  of  the  elongate 
gastrula  for  which  I  was  searching,  I  conclude  that  both  Nus¬ 
baum  and  Cholodkovsky  may  have  been  deceived  by  the  carina 
on  the  ventral  surface,  which,  in  reflected  light,  looks  like  a 
narrow  groove. 

The  method  of  germ-layer  formation  in  Blatta,  at  first  sight 
so  different  from  the  method  observed  in  DorypJiora,  may,  how¬ 
ever,  be  traced  to  the  typical  process.  My  studies  on  Dory¬ 
pJiora  make  it  probable  that  the  entoderm  of  Blatta  originates 
in  the  mass  of  cells  found  under  the  area  of  proliferation,  the 
more  superficial  cells  of  which  grow  forward  as  the  mesoderm 
in  a  continuous  median  sheet  bifurcating  under  the  cephalic 
portion  of  the  ventral  plate.  We  should  thus  have  a  spot  differ¬ 
ing  in  no  essential  particular  from  the  caudal  plate  of  Dory- 
phora.  If  we  suppose  that  in  Blatta  the  tendency  to  form  the 
median  groove  has  become  very  weak,  and  is  now  confined  to 


No.  2.] 


BLATTA  AND  DORYPHORA. 


339 


the  posterior  end  of  the  ventral  plate,  we  may  regard  the  de¬ 
pression  bp  as  the  remains  of  the  blastopore.  I  have  remarked 
that  the  gastrula  in  Dorypliora  is  much  deeper  posteriorly  than 
anteriorly,  and  providing  a  tendency  to  obliterate  the  invagina¬ 
tion  should  become  apparent  in  an  equal  degree  throughout  its 
whole  length,  a  short  posterior  depression  like  that  in  Blatta 
would  be  the  result.  But  the  opposite  possibility,  viz.  :  the 
derivation  of  a  gastrula  like  that  of  DorypJiora  from  a  circular 
form  like  that  of  Blatta,  is  likewise  worthy  of  attention.  Ac¬ 
cording  to  Sedgwick  (43),  the  gastrula  of  Peripatus  elongates 
with  a  concomitant  closure  of  the  median  portion  of  its  orifice. 
Of  the  two  openings  thus  formed  the  anterior  becomes  the 
mouth  while  the  posterior  becomes  the  anus  of  the  embryo. 
Providing  a  similar  stretching  of  the  gastrula  has  taken  place 
in  the  ancestors  of  DorypJiora  and  HydropJiiliis,  it  would  be 
easy  to  see  how  the  cells  of  the  original  single  Entodermanlage 
might  be  separated  to  form  two  masses,  which  now  arise  beneath 
the  stomodaeal  and  proctodaeal  area,  and  how  the  formation  of 
the  mesoderm  from  the  edge  of  what  was  once  the  gastrula  lips 
might  continue  throughout  the  whole  portion  of  the  embryo 
between  the  mouth  and  anus.  If  this  has  been  the  true 
evolutionary  process  in  the  development  of  the  elongate  gas¬ 
trula  of  insects,  it  seems  probable  that  Blatta  may  represent 


Fig.  I. 


Fig.  2. 


Figure  i.  —  Diagram  of  germ-layer  formation  in  DorypJiora.  vp.  ventral  plate; 
og.  oral  end  of  gastrula;  ag.  anal  end  of  gastrula;  g.  central  portion  of  gastrula;  en. 
entoderm;  e.  prolongations  of  entoderm;  aa.  plane  of  cross-section  A\  bb.  of  cross- 
section  B',  cc.  of  cross-section  C;  ec.  ectoderm  of  ventral  plate;  ms.  mesoderm. 

Figure  2.  —  Diagram  of  germ  layer  formation  in  Blatta  (somewhat  hypothetical). 
Letters  the  same  as  in  Figure  i. 


340 


WHEELER. 


[VoL.  nr. 


the  more  primitive  and  Doryphora  the  more  modified  method 
of  germ-layer  formation.  I  would  note  in  this  connection  that 
the  mesoderm  of  Blatta  is  formed  in  a  manner  strikingly  sim¬ 
ilar  to  that  observed  in  PeripaUis  by  Kennel  (24).  Reference 
to  the  diagrams,  Figs,  i  and  2,  will  make  further  remarks  on  the 
relation  of  the  two  modes  of  germ-layer  formation  in  Blatta  and 
Doryphora  unnecessary. 

Kowalevsky,  Hatschek,  Patten,  Heider,  and  Biitschli  have 
published  observations  which  have  a  decided  bearing  on  the 
method  of  entoderm  formation  in  Doryphora. 

Kowalevsky  (26)  claimed  in  his  epoch-making  work  that  in 
Hydrophiliis  the  two  longitudinal  bands  of  entoderm  are  derived 
by  delamination  from  the  splanchnic  mesoderm,  which  he  erro¬ 
neously  supposed  to  originate  from  the  primitive  mesodermic 
layer  by  an  incurling  of  its  lateral  edges.  As  we  have  seen, 
Cholodkovsky  also  claims  that  the  entoderm  of  Blatta  is  derived 
by  delamination  from  the  splanchnic  mesodern. 

Hatschek  (18)  figures  in  Bombyx  chrysorrhoea  a  large  mass 
of  entoderm  cells  immediately  beneath  the  stomodaeum.  This 
mass  may  be  compared  with  either  the  anterior  or  posterior  cell 
masses  of  a  similar  nature  in  Doryphora. 

Patten  (38)  figures  a  cluster  of  several  huge  entoderm  cells 
attached  to  the  stomodaeum  of  Neophalax  (PL  XXIV.  C.  Fig. 
36),  and  in  a  more  recent  paper  (39)  he  has  figured  similar  cells 
in  the  same  position  in,  the  embryo  Aciliiis. 

Two  recent  papers  on  Mtisca,  one  by  Kowalevsky  (27)  and 
one  by  Biitschli  (8),  contain  accounts  of  a  method  of  entoderm 
formation  very  similar  to  that  observed  by  me  in  Doryphora. 

Kowalevsky  finds  that  the  entoderm  originates  in  two  widely 
different  points,  anteriorly  at  the  inner  end  of  the  stomodaeum 
and  posteriorly  at  the  inner  end  of  the  proctodaeum,  from  some 
of  the  cells  of  the  gastrular  invagination.  The  mass  of  entoderm 
at  either  of  these  places  forms  a  watch-glass-shaped  body  with 
its  concavity  applied  to  the  yolk.  From  the  lateral  edges  of 
each  mass  the  entoderm  cells  proliferate  to  form  two  bands, 
each  of  which  unites  with  the  one  on  the  same  side  growing 
from  the  opposite  direction.  By  a  dorsal  and  ventral  growth  of 
the  edges  of  the  two  bands  the  entoderm  envelops  the  yolk  and 
thus  completes  the  mesenteron.  Kowalevsky  also  regards  the 
entoderminic  invagination  in  insects  as  a  greatly  elongated  gas- 


No.  2.J 


BLATTA  AND  DORVPHORA. 


34r 


trula,  which  has  retained  its  ability  to  form  entoderm  only  at  the 
oral  and  anal  ends.  Putting  this  construction  on  the  gastrula, 
it  is,  of  course,  easy  to  reduce  the  germ  layers  of  insects  to  the 
Sagitta  pattern. 

Kowalevsky’s  results  on  Mtisca  have  been  corroborated  as 
far  as  the  posterior  Entodernianlage  is  concerned  by  Biitschli 
(8)  ;  while  his  main  results  on  Hydrophihis  have  been  con¬ 
firmed  by  Heider  (19). 

According  to  Heider,  the  tube  formed  by  the  closing  gastrula 
flattens  out,  and  the  half  of  it  immediately  below  the  ectoderm 
becomes  mesoderm,  afterwards  splitting  into  the  somatic  and 
splanchnic  layers ;  while  the  other  (inner)  half  becomes  the 
entoderm  which  spreads  apart  to  form  two  bands,  one  on  each 
side  applied  to  the  part  of  the  mesoderm  inclosing  the  coelomic 
cavity.  Subsequently  the  proliferating  edges  of  the  bands 
unite  ventrally  and  dorsally  to  complete  the  mesenteron. 

A  comparison  of  my  account  of  DorypJiora  with  H eider’s 
account  of  Hydrophihis  will  show  that  I  differ  from  Heider  on 
one  point  only.  I  claim  that  all  the  entoderm  between  the  oral 
and  caudal  widenings  of  the  blastopore  is  not  derived  from  the 
inner  cells  of  the  gastrular  depression,  but  grows  in  from  the 
ends  of  the  body.  I  do  not  deny  that  the  entoderm  may  arise 
in  Hydrophihis  (and  possibly  in  Doryphord)  in  the  manner 
described  by  Heider,  i.e.  from  the  inner  layer  of  cells  of  the 
gastrular  tube  when  it  flattens  out  and  breaks  down,  but  I 
would  regard  the  process  as  confined  to  two  very  small  areas, 
one  being  stomodaeal,  the  other  protodaeal. 

Heider’s  figures  are  undoubtedly  correct  and  correspond  in 
every  way  to  sections  through  Doryphora  embryos  in  corre¬ 
sponding  stages.  Unfortunately,  he  has  not  noted  with  any 
precision  the  plane  of  section  of  the  different  preparations 
figured.  Starting  with  his  Plate  II.,  his  figures  may  be  inter¬ 
preted  in  harmony  with  Doryphora,  thus  :  — 

Figure  23  is  exactly  like  my  Fig.  88,  omitting  the  degenerating 
nuclei,  and  I  should  interpret  it  in  the  same  way  as  Heider. 
My  section  passes  through  the  caudal  plate  of  the  embryo ; 
Heider’s  passes  “durch  den  Abdominaltheil,”  which  is  indefi¬ 
nite.  In  Figs.  24  and  25  I  can  see  no  entoderm,  but  merely 
the  apposed  splanchnic  and  somatic  layers  of  mesoderm  between 
the  lateral  ends  of  which  the  coelomic  cavities  are  about  to 


342 


WHEELER. 


[VOL.  III. 


form,  these  cavities  being  simply  enlargements  of  the  at  first 
very  limited  space  between  the  two  layers  at  their  outer  extremi  ¬ 
ties.  Figure  25  is  a  section  through  the  “  Abdominaltheil.” 

Both  figures  accurately  represent  cross-sections  through  the 
middle  of  the  ventral  half  of  the  egg  of  Doryphora  before  the 
proliferating  bands  of  entoderm  have  reached  the  plane  of 
section  (compare  my  Fig.  78).  Heider’s  Fig.  26  just  skims  the 
ends  of  the  proliferating  bands,  showing  four  entoderm  cells  at 
i  beneath  the  left  coelomic  cavity  in  the  figure.  No  portion  of 
the  band  was  cut  on  the  opposite  side,  as  either  the  section  was 
slightly  oblique  or  one  of  the  bands  had  grown  somewhat  more 
rapidly  than  the  other,  a  condition  which  I  have  often  observed 
in  DorypJiora. 

The  most  definite  proof  that  Hydrophihis  does  not  differ  from 
DorypJiora  is  to  be  gained  from  Heider’s  own  words.  He  says  : 
“Wir  miissen  aber  nun  auf  ein  hbchst  merkwiirdiges  Verhalten 
eingehen,  welches  uns  beweist,  bis  zu  welchem  Grade  coenoge- 
netische  Veranderungen  den  urspriinglichen  Typus  der  Insecten- 
entwicklung  entstellen.  Wahrend  namlich  die  geschilderte 
Abtrennung  des  Entoderms  von  dem  unteren  Blatte  Kowalev- 
sky’s  im  vorderen  Theil  des  Hydrophilus-Embryos  (den  Kopf- 
und  Thoraxsegmenten)  deutlich  zu  beobachten  ist  und  ebenso 
klar  in  den  letzten  Abdominalsegmenten  zur  Ausbildung 
kommt,  treffen  wir  entsprechend  den  vorderen  Segmenten  des 
Abdomens  eine  Ouerzone  des  Embryos,  in  welcher  keine  Ento- 
dermchicht  zur  Anlage  kommt  —  mit  anderen  Worten :  die 
Entodermanlage  entwickelt  sich  im  Vordertheil  und  nahe  dem 
Hinterende  des  Embryos  in  zwei  gesonderten  Stiicken,  welch  e 
erst  in  spateren  Stadien  gegeneinander  wachsen  und  mit  einan- 
der  verschmelzen.  Diese  gesonderte  Ausbildung  des  Entoderms 
vom  Vorder  und  Hinterende  des  Embryos  ist  ein  Seitenstiick 
zu  dem  von  uns  geschilderten  und  (Taf.  L.  Fig.  4)  abgebildeten, 
selbstandigen  Auftreten  des  Vorder-und  Hinterendes  der  rin- 
nenformigen  Einstiilpung.  Wie  ich  aus'den  Angaben  Kowalev- 
sky’s  and  Grassi’s  ersehe,  weist  das  Darmdriisenblatt  der  Biene 
hinsichtlich  seiner  ersten  Anlage  ahnliche  Verhaltnisse  auf.” 

b.  Embryonic  Envelopes. 

No  phenomenon  in  the  development  of  the  insect  embryo  is 
better  suited  to  call  forth  conjecture  than  the  embryonic  enve- 


No.  2.] 


BLATTA  AND  DORYPHORA. 


343 


lopes  and  the  dorsal  organ  formed  soon  after  their  rupture. 
For  the  sake  of  clearness  I  shall  here  consider  only  the  envel¬ 
opes  and  relegate  the  discussion  of  the  dorsal  organ  to  my 
remarks  on  the  revolution  of  Blatta  and  Doryphora  at  the  end 
of  the  next  descriptive  division  of  my  subject. 

Various  theories,  all  more  or  less  vague  and  intangible,  have 
been  advanced  by  different  investigators  to  account  for  the 
amnion  and  serosa.  Balfour  (2)  regarded  these  membranes  as 
possibly  derived  from  an  early  ecdysis.  Ayers  (i)  refuted 
Balfour’s  suggestion ;  but  as  he  started  out  in  his  own  explana¬ 
tion  with  incorrect  suppositions  regarding  the  homologies  of 
the  different  germ-layers  in  insects  with  those  of  other  animals, 
he  could  not  fail  to  involve  the  amnion  and  serosa  in  the  gen¬ 
eral  error.  Kennel  (24)  regards  the  embryonic  membranes  of 
insects  as  homologous  with  the  so-called  “amnion”  in  Peripatiis^ 
and  both  structures  as  the  remains  of  the  trochosphere  of  the 
annelid  ancestor.  Emery  (ii)  suggests  that  the  envelopes  may 
be  homologous  with  the  shell  of  the  Entomostraca. 

The  question  as  to  the  meaning  of  the  envelopes  in  insects 
has  been  greatly  confused  by  drawing  in  the  widely  different 
envelopes  of  Peripatus,  ScorjDions  and  Myriopods  and  the  Crus¬ 
tacean  dorsal  organ,  presenting  all  the  different  forms  observed 
in  Oniscus,  Asellus,  Cyinothoa^  Mysis,  etc.  Some  authors  agree 
with  Kennel  in  regarding  the  embryonic  envelopes  throughout 
the  Arthropoda  as  homologues.  According  to  others  the  dorsal 
organs  of  the  Crustacea  are  the  homologues  of  the  amnion  and 
serosa  of  hexapods.  Still  others  maintain  that  the  Crustacean 
dorsal  organ  is  to  be  brought  into  connection  with  the  occasion¬ 
ally  similar  dorsal  organ  of  insects. 

Will  (52)  has  of  late  advanced  a  theory  to  account  for  the 
formation  of  the  embryonic  envelopes  of  insects  only.  His 
theory  has  the  advantage  over  its  precursors  in  that  it  replaces, 
such  indefinite  terms  as  “early  ecdysis,”  “shell  of  the  Entomo-. 
straca,”  and  “Trochosphere”  by  facts  derived  from  the  com¬ 
parative  morphology  of  the  membranes  themselves.  As  I  came 
to  essentially  the  same  conclusions  as  Will  long  before  reading 
his  article,  I  may  be  pardoned  for  presenting  the  subject  in  my 
own  words,  though  they  repeat  in  great  measure  what  has 
appeared  in  Will’s  paper. 

The  problem  as  to  the  meaning  of  the  amnion  and  serosa  is 
restricted  to  the  Hexapoda  by  postulating  the  following  :  — 


344 


WHEELER. 


[VOL.  III. 


1.  There  are  no  sufficient  reasons  for  homologizing  the 
embryonic  envelopes  of  insects  with  the  homonymous  but 
dissimilar  structures  in  Myriopods,  Scorpions,  and  Peripatus. 

2.  There  is  no  more  than  a  superficial  resemblance  to  speak 
for  an  homology  between  the  dorsal  organs  of  the  Crustacea  and 
the  embryonic  envelopes  of  Insects  or  between  the  dorsal 
organs  of  Crustacea  and  the  homonymous  structures  in  Insects. 

3.  The  dorsal  organ  of  insects  may  be  regarded  as  the  neces¬ 
sary  result  of  the  rupture  and  absorption  of  the  embryonic 
envelopes,  and  consequently  as  in  no  way  related  to  such 
structures  as  the  dorsal  organs  of  Cymotlwa,  Liniuhts,  etc. 

The  process  of  envelope  formation  has  been  observed  in 
numerous  insects  of  all  orders  with  sufficient  accuracy  to  war¬ 
rant  the  assertion  that  all  the  pronounced  types  are  connected 
by  intermediate  forms  in  fine  gradation  ;  a  fact  which  was  long 
ago  expressed  by  Kowalevsky  (26). 


Fig.  6.  Fig.  7.  Fig.  8. 


Diagrammatic  longitudinal  sections  of  embryos  just  after  the  completion  of  the 
envelopes.  Figure  g,  Geophilus ;  Figure  4,  Calopteryx ;  Figure  g,  Aphis;  Figure 
6,  Doryphora ;  Figure  7,  Blatta ;  Figure  8,  Bombyx.  a.  amnion;  s.  serosa;  as. 
amnion  and  serosa  apposed;  vp.  ventral  plate;  pd.  procephalic  lobes;  y.  yolk. 

If  we  suppose  such  a  form  as  Calopteryx  to  present  the  origi¬ 
nal  mode  of  embryo  formation  in  the  Hexapoda  (and  Paleontol- 


No.  2.] 


BLATTA  AND  DORYPHORA. 


345 


ogy  makes  such  a  supposition  probable),  we  have  a  form  which 
will  unite  readily  with  an  embryo  Myriopod  in  a  corresponding 
stage  of  development.  In  Fig.  3  I  have  given  a  diagrammatic 
longitudinal  section  of  Geophilus  about  the  time  the  appendages 
appear.  Evidently  either  the  excessive  growth  in  length  of  the 
annelid-like  body  has  necessitated  the  complete  invagination  of 
the  embryo  into  the  yolk,  bringing  the  caudal  and  cephalic  ends 
together,  or  this  process  has  been  adopted  as  a  means  of  bring¬ 
ing  the  surface  of  the  embryo  into  more  complete  contact  with 
the  yolk.  It  is  only  necessary  to  reduce  the  posterior  half  of 
the  infolded  embryo  (included  between  x  and  m  in  the  figure) 
to  a  thin  membrane  to  reach  the  condition  of  Calopteryx  (Fig.  4). 
The  membrane  resulting  from  the  attenuation  would  be  the 
amnion.  Will  has  emphasized  the  fact  that  the  stage  with  the 
amnion  almost  as  thick  as  the  ventral  plate  with  which  it  is 
continuous,  still  occurs  in  the  ontogeny  of  insects  (compare 
Blatta  and  NeopJialax).  He  suggests  that  the  attenuation  of 
the  posterior  half  of  an  embryo  like  Geophilus  to  form  the 
amnion  may  account  for  the  great  disparity  in  the  number  of 
segments  between  the  Hexapoda  and  Myriopoda. 

A  further  difference  is  observable  between  Calopteryx  and 
Geophilus.  The  end  of  the  original  caudal  extremity  in  the 
former  is  joined  to  the  anterior  end  of  the  ventral  plate,  thus 
closing  the  sack  whose  anterior  wall  is  the  ventral  plate  and 
whose  posterior  wall  is  the  amnion.  This  sack  is  attached  at 
one  point  to  the  serosa  enveloping  the  egg.  The  union  of  the 
membranes  to  close  completely  the  amniotic  cavity  is  the  hinge 
about  which  the  further  explanation  turns. 

It  seems  essential  in  all  the  insects  so  far  studied  excepting 
Musca,  where  the  envelopes  are  rudimentary,  that  the  amniotic 
cavity  should  be  shut  off  from  the  space  between  the  vitelline 
membrane  and  the  surface  of  the  yolk.  The  reason  for  the 
closure  is  apparent  if  we  regard  the  amniotic  cavity  as  a 
place  for  the  temporary  deposition  of  excreted  matters,  as  an 
organ  functionally  analogous  to  the  allantois  of  higher  animals. 
It  has  often  been  observed  that  the  amniotic  cavity  of  insects 
soon  after  its  formation  becomes  filled  with  a  clear  liquid  which 
during  and  after  revolution  is  found  as  a  much  vacuolated  coagu- 
lum  about  the  feet  of  hardened  embryos.  It  seems  probable  that 
while  the  inner  ends  of  the  ventral-plate  cells  are  absorbing  and 


346 


WHEELER. 


[VOL.  III. 


metabolizing  the  yolk,  their  outer  ends  are  at  the  same  time 
giving  off  into  the  amniotic  cavity  a  less  amount  of  liquid  waste 
products.  Providing  this  supposition  is  true,  we  should  have  a 
sufficient  reason  for  the  constant  closure  of  the  amniotic  cavity. 

We  have  a  complete  series  of  finely  graduated  forms  of  enve¬ 
lope  formation  from  the  method  observed  in  Calopteryx  (Fig.  4) 
to  Blatta  (Fig.  7).  Aphis  represents  the  first  step  in  the  tran¬ 
sition  of  an  e7itoblastic  embryo  like  Calopteryx  to  the  decidedly 
ectoblastic  form  seen  in  Blatta.  This  transition  consists  in 
leaving  more  and  more  of  the  anterior  end  of  the  embryo  on 
the  surface  of  the  yolk.  In  Aphis  (Fig.  5)  the  whole  head  is 
left  outside  the  invagination  ;  in  Doryphora,  the  head  and  the 
anterior  half  of  the  body.  When  a  portion  of  the  embryo  is  left 
on  the  surface,  the  closure  of  the  amniotic  cavity  necessitates  a 
backward  growth  of  the  angle  formed  by  the  fore  end  of  the 
head  and  the  abutting  serosa  {aSy  Fig.  5)  to  form  a  fold  which 
unites  with  a  similar  fold  formed  at  the  opposite  end  of  the 
embryo.  In  Doryphoray  where  much  of  the  embryo  lies  on  the 
surface  of  the  yolk,  the  posterior  or  caudal  fold  of  the  amnion 
and  serosa  has  to  grow  forward  a  considerable  distance  to  meet 
the  cephalic  fold.  Blatta  has  advanced  still  further  than  Dory- 
phora.  The  embryo  no  longer  grows  into  the  yolk,  but  the 
formation  and  ultimate  closing  of  the  membranes  continues. 

At  first  sight  it  would  seem  more  natural  to  suppose  that  the 
result  attained  in  Blatta  was  brought  about  simply  by  an  extru¬ 
sion  of  the  yolk  between  the  amnion  and  serosa  of  such  a  form 
as  Aphis  or  Calopteryx y  but  the  law  of  orientation,  as  explained 
in  a  preceding  paragraph,  forbids  such  an  interpretation.  The 
head  of  the  Aphis  embryo  is  at  the  time  of  the  completion  of 
the  membranes  close  to  the  spot  before  occupied  by  the  caudal 
end  of  the  ventral  plate,  and  after  revolution  the  caudal  end  of 
the  embryo  will  again  be  located  at  this  end  of  the  egg.  Hence 
the  typical  ectoblastic  originated  from  the  typical  entoblastic 
embryo,  not  by  an  extrusion  of  the  yolk  from  between  the 
amnion  and  serosa,  but  by  a  gradual  weakening  of  the  invagi- 
native  process.  The  weakening,  of  course,  results  in  more  and 
more  of  the  anterior  portion  of  the  ventral  plate  remaining  inert, 
though  the  growth  of  the  membranes  to  shut  off  the  amniotic 
cavity  continues. 

The  peculiar  free  entoblastic  embryo  observed  in  Lepidoptera 


INTO.  2.] 


BLATTA  AJVD  DORYPHORA. 


347 


(Fig.  8)  may  have  originated  in  two  ways  :  either  from  a  sus¬ 
pended  entoblastic  embryo  like  Calopteryx  by  a  separation  of 
the  amnion  from  the  serosa  at  the  point  of  suspension  (Fig.  /\as) 
and  the  consequent  passage  of  yolk  between  the  two  membranes, 
or  from  the  ectoblastic  type  of  Blatta  by  a  separation  of  the 
amnion  from  the  serosa  throughout  their  area  of  contact,  accom¬ 
panied  by  an  intrusion  of  yolk.  All  that  we  at  present  know 
concerning  the  formation  of  the  envelopes  in  Lepidoptera  tends 
to  prove  that  the  latter  method  is  the  more  probable. 

The  hypothesis  of  Will  and  myself  as  set  forth  in  the  above 
paragraphs  might  be  called  a  mechanical  explanation  as  opposed 
to  the  views  of  those  who  see  in  the  embryonic  membranes 
rudimental  structures  like  the  remains  of  the  trocosphere,  larval 
skins,  etc.  Our  hypothesis  has  at  least  the  virtue  of  utilizing 
the  facts  near  at  hand. 

Description  of  the  External  Changes  in  the  Embryo 

Blatta  and  Doryphora  up  to  the  Time  of  Hatching. 

Blatta. 

Soon  after  their  completion,  the  amnion  and  serosa  become 
more  attenuated  on  account  of  the  flattening  of  their  cells  and 
the  consequent  diastasis  of  their  nuclei.  This  thinning  out  of 
the  envelopes  permits  a  better  view  of  the  embryo  and  its  form¬ 
ing  appendages. 

On  about  the  tenth  or  eleventh  day  from  the  beginning  of 
development,  the  embryo  presents  the  appearance  represented 
in  Fig.  45.  The  broadly  rounded  procephalic  lobes  are  separated 
by  a  deep  incision  in  the  median  line,  and  the  antennae  {at) 
growing  from  the  posterior  lateral  corners  of  the  lobes  have 
become  prominent,  while  the  backward  direction  of  their  growth 
is  apparent.  The  labrum  {lb)  has  appeared  as  a  thick,  crescen¬ 
tic  and  slightly  divided  fold  in  front  of  a  faint  depression 
which  is  the  commencement  of  the  stomodaeal  invagination.  Of 
the  three  pairs  of  oral  appendages,  the  second  and  third  {mx^,  mx^) 
are  clearly  rounded  and  directed  backwards  ;  the  mandibles  are 
still  small.  Each  segment  of  the  abdomen  presents  a  pair  of 
indistinct  appendages.  These  subsequently  disappear,  with  the 
exception  of  the  pairs  on  the  basal  and  terminal  segments, 
which  undergo  a  differentiation  peculiar  to  themselves.  As  I 


348 


wheeler. 


[VOL.  III. 


have  given  elsewhere  (50)  a  minute  account  of  the  pair  of  appen¬ 
dages  belonging  to  the  first  abdominal  segment,  I  shall  not  con¬ 
sider  them  in  the  present  paper.  The  pair  of  appendages  belong¬ 
ing  to  the  terminal  segment  persist  and  become  the  anal  stylets. 
Unlike  most  Arthropod  embryos,  the  caudal  end  of  the  embryo 
Blatta  {cpl)  is  never  bent  dorsally,  but  from  the  very  early  stage 
in  which  it  is  in  a  line  with  the  long  axis  of  the  abdomen,  shows 
only  a  ventral  flexure.  Owing  to  this  flexure,  which  soon  be¬ 
comes  very  pronounced,  the  formation  of  the  protodoeum  cannot 
be  as  easily  observed  as  in  Doryphora. 

The  changes  which  are  apparent  in  surface  views  by  the  four¬ 
teenth  or  fifteenth  day  have  been  carefully  represented  in  Figs. 
46  and  47.  In  the  former  the  embryo  is  in  situ  on  the  yolk,  in 
the  latter  it  is  isolated  and  seen  from  the  ventral  surface. 

As  may  be  indistinctly  seen  in  Fig.  47,  the  first  and  second 
maxillary  appendages  have  each  become  split  up  into  three  divis¬ 
ions.  In  QLcantJiuSy  according  to  Ayers,  “  the  three  oral  appen¬ 
dages  are  trilobed  ;  the  lobation  is  most  prominent  in  the  second 
maxillary,  and  least  in  the  mandibular  appendages.  The  primi¬ 
tive  appendage  is  first  divided  into  two  lobes,  and  the  inner  of 
these  becomes  secondarily  divided  into  two.”  There  are  appar¬ 
ently  no  traces  of  lobation  in  the  mandibles  of  Blatta.  The 
outer  of  the  three  lobes  of  each  maxilla  becomes  the  palp,  while 
the  inner  two  become  the  galea  and  lacinia  of  the  adult. 


Fig.  g.  Fig.  10.  Fig.  ii.  Fig.  12.  Fig.  13. 


Figure  g.  —  Embryo  of  Blatta,  15  days  old;  revolution  about  to  begin.  The 
stages  in  revolution  are  represented,  after  the  rupture  of  the  amnion  and  serosa,  in 
Figures  10-13,  which  are  from  embryos  16,  1 6^,  i6|,  and  17  days  old  respectively. 
as.  amnion  and  serosa;  s.  edge  of  serosa;  b.  dorsad  growing  body  wall;  d.  0.  dorsal 
organ;  x.  clear  zone  covered  with  scattered  amniotic  nuclei. 


No.  2.]' 


BLATTA  AND  DORYPHORA. 


349 


In  Blatta  the  formation  of  the  nervous  system  in  its  earlier 
stages  cannot  be  clearly  seen  from  the  exterior.  The  same 
holds  true  of  the  small  tracheal  invaginations,  though  several 
pairs,  especially  those  of  the  thorax  and  basal  abdominal  rings, 
may  be  seen  on  the  pleurae  in  good  preparations  before  revolu¬ 
tion.  Still  they  are  so  much  less  distinct  than  in  Doryphora 
that  I  have  given  them  little  attention. 

The  peculiar  phenomena  of  revolution  are  hurried  through  by 
the  embryo  from  the  beginning  of  the  sixteenth  to  the  end  of 
the  seventeenth  day.  Several  successive  stages  in  the  process 
are  represented  in  the  woodcuts  (Figs.  9  to  13).  When  fifteen 
days  old  (Fig.  9)  the  embryo  still  occupies  the  middle  of  the 
ventral  surface  of  the  egg,  the  distance  from  the  head  to  the 
cephalic  end  of  the  yolk  being  almost  equal  to  the  distance  of 
the  tail  from  the  caudal  end  of  the  yolk.  The  amnion  and 
serosa  {as)  still  envelop  the  embryo,  though  they  have  become 
much  attenuated.  By  the  end  of  the  fifteenth  or  the  beginning 
of  the  sixteenth  day,  the  envelopes  rupture,  an  irregular  slit 
being  formed  down  the  median  ventral  line.  The  amnion  now 
appears  to  undergo  degeneration,  at  least  in  part,  while  the 
serosa  is  drawn  back  from  both  sides  by  a  contraction  of  the 
protoplasm  of  its  cells,  the  large  nuclei  of  which  make  it  easy 
to  trace  all  the  steps  in  the  formation  of  the  dorsal  organ. 

Soon  after  the  rupture  of  the  envelopes  the  embryo  and  egg, 
when  seen  from  the  side,  resemble  Fig.  10.  The  embryo  stands 
out  free  from  its  envelopes  on  the  yolk ;  the  edges  of  its  dorsad 
growing  walls  {b)  are  distinctly  marked.  Near  these,  on  the  sur¬ 
face  of  the  egg,  are  seen  a  number  of  scattered  nuclei,  which  are 
of  the  same  size  as  the  nuclei  of  the  cells  forming  the  embryo. 
These  I  take  to  belong  to  the  portion  of  the  amnion  which  has 
become  folded  back  on  the  yolk  and  forms  a  zone  w  extending 
the  whole  length  of  the  yolk  in  contact  with  the  dorsad  growing 
body  wall.  Next  to  this  zone  lies  another  zone,  which  is  bounded 
by  the  distinct  edge  of  the  serosa  {s),  and  which  I  regard  as  a 
portion  of  the  yolk  left  bare,  as  no  nuclei  are  to  be  found  on  its 
surface.  Besides  the  ventrodorsal  contraction  in  the  substance 
of  its  anterior  edge  {s),  the  serosa  contracts  in  an  antero-poste- 
rior  direction,  thus  producing  the  constriction  seen  at  j/  in  Fig. 
10.  The  rounded  and  projecting  lump  formed  at  the  caudal 
pole  is  the  beginning  of  the  dorsal  organ  (Fig.  10  d.  0).  The 


350 


WHEELER. 


[VoL.  nr. 


contraction  continues  towards  the  median  dorsal  line  and  towards 
the  cephalic  pole.  In  Fig.  ii  the  dorsal  organ  has  moved  half¬ 
way  up  the  dorsal  surface.  Its  darker  color  in  stained  embryos 
is  due  to  the  fact  that  the  serosal  cells  have  become  deeply 
columnar  with  a  consequent  approximation  of  their  large  nuclei 
to  one  another.  The  embryo,  besides  increasing  in  size,  has 
undergone  a  change  in  position.  Its  tail  now  lies  at  the  caudal 
end  of  the  egg.  Notwithstanding  the  embryo’s  growth  in  length, 
its  head  lies  much  lower  than  in  the  preceding  stage  (Fig.  lo). 
The  body  wall  {b)  is  still  distinct,  the  zone  of  sporadic  amni- 
otic  (i’)  nuclei  {x)  has  increased  in  breadth.  As  in  the  preceding 
stages,  these  nuclei  are  most  closely  aggregated  near  the  edge 
of  the  advancing  body  wall.  In  the  next  stage  (Fig.  12)  the 
dorsal  organ  has  reached  the  cephalic  end  of  the  yolk  and  bulges 
out  like  a  large  hood.  The  body  walls  of  the  embryo  have 
nearly  enveloped  the  yolk  at  the  caudal  end. 

The  next  change  takes  place  very  rapidly.  The  stage  repre¬ 
sented  in  Fig.  12  is  attained  towards  the  end  of  the  sixteenth 
day.  By  the  seventeenth  day  the  walls  have  closed  in  the 
median  dorsal  line,  and  the  embryo  has  grown  in  length  to  such 
an  extent  as  to  bring  its  head  to  the  cephalic  pole.  The  dorsal 
organ  has  been  shut  in  by,  and  lies  immediately  below,  that  por¬ 
tion  of  the  body  wall,  which  will  form  the  tergum  of  the  pro¬ 
thorax.  On  entering  the  yolk  the  cells  of  the  dorsal  organ  begin 
to  disintegrate.  Two  of  the  stages  in  the  formation  and  disso¬ 
lution  of  the  dorsal  organ  are  represented  in  Figs.  50  and  51, 
both  from  longitudinal  sections,  the  former  being  sagittal,  the 
latter  frontal. 

Figure  50  represents  a  section  through  the  centre  of  the  thick¬ 
ened  mass  of  serosal  cells.  The  deeply  stained  nuclei  are  seen 
crowded  together  in  the  inner  ends  of  the  cells,  the  contours  of 
which  are  rendered  papillose  apparently  by  the  pressure  of  the 
nuclei  against  the  cell  walls.  The  outer  ends  of  the  elongated 
columnar  cells  form  a  thick  layer  of  granular  protoplasm  con¬ 
siderably  depressed  at  0.  This  depression  is  equivalent  to  the 
tubular  cavity  in  the  dorsal  organ  of  Hydrophilus. 

In  Fig.  51  the  large  lump  of  cells  has  become  engulfed  in  the 
yolk.  The  body  wall  has  closed  over  it,  and  the  heart  {cc)  has 
formed  between  it  and  the  ectoderm.  The  large  deeply  staining 
nuclei  {7171)  are  seen  in  the  various  stages  of  active  degeneration, 


No.  2.] 


BLATTA  AND  DORYPHORA. 


351 


many  of  which  recall  the  degenerating  nuclei  in  the  entoderm 
of  DorypJiora.  We  have  the  same  vacuolization  of  the  karyo- 
plasm  and  agglomeration  of  the  ehromatin.  In  the  centre  of 
the  mass  a  pale,  oval  spot  surrounds  an  elongated  cavity  {p), 
which  is  almost  obliterated.  This  cavity  results  from  the  de¬ 
pression  0  of  Fig.  50  by  a  closing  in  over  it  of  the  peripheral 
edges  of  the  dorsal  organ. 

Figure  48,  drawn  from  an  advanced  embryo  saturated  with 
clove  oil,  shows  the  condition  of  the  different  organs  shortly 
before  hatching.  The  embryo  preserves  the  shape  of  the  egg, 
being  much  flattened  laterally.  The  segments  of  the  body  are 
all  distinctly  defined.  The  mouth  parts  have  become  closely 
approximated,  and  have  assumed  their  definite  relations  to  one 
another.  The  long  antennae  {at)  extend  as  far  as  the  two  anal 
stylets  {ast)  in  which  the  ventrally  bent  tip  of  the  abdomen 
terminates.  The  different  divisions  of  the  alimentary  canal, 
oesophagus  {pe),  ingluvies  (c),  proventriculus  {gz),  stomach  {st)^ 
still  containing  the  remains  of  the  yolk  with  its  degenerating 
nuclei,  and  rectum  {ret),  ending  in  the  anus  posteriorly,  and 
surrounded  by  a  wreath  of  Malpighian  vessels  {nipg)  anteriorly, 
may  be  readily  traced  in  the  figure.  The  heart  is  seen  as  a 
delicate  tube  just  beneath  the  dorsal  integument.  The  large 
supraoesophageal  ganglion  {cgl)  connected  with  the  large  lateral 
compound  eyes,  in  which  the  pigment  is  being  deposited,  fills 
the  greater  portion  of  the  brain-box.  One  of  the  commissures 
is  seen  connecting  it  with  the  inf  race  sophageal  ganglion  (^/^). 
The  three  thoracic  ganglia  (^/^,  gl‘^,  gl^)  are  much  larger  than 
the  six  abdominal  ganglia.  A  large,  granular,  fat  body  {ad)  is 
applied  to  the  inner  surface  of  pleural  wall  of  the  abdomen. 
The  refractive  granules  imbedded  in  it  form  a  chevron  in  each 
of  the  first  five  or  six  somites.  Patten  (38)  in  his  preliminary 
note  on  Blatta  thus  describes  the  physical  and  chemical  nature 
of  these  bodies :  “  In  the  embryos  of  Blatta,  as  well  as  in  those 
of  most  if  not  all  other  insects,  there  appears  in  each  of  the 
segments  at  a  certain  time  a  great  number  of  clear,  highly 
refractive  particles  that  at  first  might  be  taken  for  oil  globules, 
and  which  have  always  been  regarded  as  such.  On  more  care¬ 
ful  examination,  however,  it  will  readily  be  seen  that  this  suppo¬ 
sition  is  incorrect.  A  number  of  tests  have  been  made  in  order 
to  ascertain  the  nature  of  these  bodies,  and  the  results  show 


352 


WHEELER. 


[VOL.  III. 


that  there  are  some  salts  of  uric  acid.  That  they  are  not  of 
a  fatty  nature  is  indicated  by  the  fact  •  that  treatment  of  the 
embryos  with  hot  benzole,  chloroform,  or  clove  oil  has  not  the 
slightest  effect  upon  the  bodies  in  question.  Further  examina¬ 
tion  with  a  high  magnifying  power  shows  that  they  consist  of 
small  spheres  of  an  extremely  refractive  substance,  from  the 
centre  of  which  dark  lines  radiate  in  an  irregular  manner,  pro¬ 
ducing  the  same  appearance  seen  in  the  crystals  of  urea  from 
the  Malpighian  vessels.  It  was  this  similarity  which  first  sug¬ 
gested  the  true  nature  of  these  bodies ;  and  further  tests  con¬ 
firmed  this  view,  for,  after  heating  an  embryo  with  nitric  acid 
upon  a  glass  slide,  and  then  adding  a  little  ammonia,  the  char¬ 
acteristic  red  color  of  Murexid  was  formed.  A  still  further  test 
was  formed  by  dissolving  the  granules  in  dilute  caustic  potash, 
and  then  precipitating  the  urea  by  adding  acetic  acid,  although 
this  method  did  not  give  such  definite  results  as  the  first.” 

The  embryonic  development  of  Blatta  is  completed  by  about 
the  thirtieth  day  from  oviposition. 

Figure  49  shows  the  embryo  soon  after  hatching.  Shortly 
after  leaving  its  narrow  place  in  the  capsule,  the  insect  under¬ 
goes  a  peculiar  change  in  shape.  While  confined  by  the  cho¬ 
rion  the  diameter  from  one  pleural  wall  to  the  other  is  about 
one-third  the  dorsoventral  diameter  of  the  insect.  Soon  after 
hatching,  its  dorsoventral  diameter  is  only  about  one-third  as 
great  as  its  greatest  breadth.  The  tip  of  the  abdomen,  ventrally 
flexed  in  the  egg,  bends  dorsally  as  indicated  by  the  position 
of  the  anal  stylets,  which  now  point  directly  upwards  and  out¬ 
wards.  The  spines  and  onychia,  most  abundant  on  the  legs, 
are  developed  shortly  before  hatching. 

Doryphora. 

Doryphora  embryos,  when  carefully  prepared,  reveal  much 
more  in  surface  views  than  Blatta  embryos  prepared  according 
to  the  same  methods. 

The  last  stage  described  is  represented  by  Fig.  73.  I  shall 
pass  over  a  few  of  the  succeeding  stages,  and  stop  to  describe 
the  embryo  represented  in  Fig.  72,  which  shows,  with  great 
clearness,  all  that  has  taken  place  in  the  omitted  stages,  and 
makes  a  description  of  them  unnecessary.  The  figure  is  slightly 


No.  2.] 


BLATTA  AND  DORVPHORA. 


353 


diagrammatic,  being  drawn  from  a  number  of  different  embryos, 
each  of  which  contributed  some  of  the  details  in  a  clearer  and 
more  pronounced  manner.  The  mouth  {o)  and  anus  {a),  both 
triangular  depressions,  have  become  clearly  established.  The 
former  has  in  front  of  it  a  heart-shaped  prominence,  the  bilobed 
labrum.  The  lateral  half  of  the  head  presents  some  interesting 
facts,  first  elucidated  in  Patten’s  paper  on  the  eyes  of  Acilms 
(39).  Each  half  of  the  head  is  divided  by  longitudinal  constric¬ 
tions  into  three  parallel  rounded  ridges,  each  of  which  is  further 
divided  by  two  transverse  depressions  into  three  subquadran- 
gular  thickenings.  The  three  inner  b^)  on  each  side,  rep¬ 

resenting  the  three  segments  of  the  brain,  are  directly  continuous 
with  the  ventral  ganglion  chain,  extending  to  the  protodseum. 
The  row  of  prominences  {og^y  og‘^y  og^)  on  the  outer  side  of  the 
three  brain  segments  are  the  optic  ganglion,  the  further  three 
{op^y  op^^y  op^)y  somewhut  indistinctly  seen  because  situated  on 
the  very  edge  of  the  head,  are  the  divisions  of  the  optic  plate, 
each  of  which,  in  AciliuSy  according  to  Patten,  bears  a  pair  of 
ocelli.  The  only  appendages  of  the  head  proper  in  this  stage 
are  the  antennae  {at)y  which  are  directed  backwards,  and  the 
heart-shaped  labrum.  The  head  also  presents  three  pairs  of 
small  invaginations  somewhat  less  distinct  than  in  the  figure 
{t^y  t'^y  t^).  These  lie  near  the  longitudinal  constriction,  sepa¬ 
rating  the  brain  thickenings  from  the  thickenings  of  the  optic 
ganglion.  This  is  best  seen  in  the  third  segment,  where  the 
invaginations  lie  at  the  bases  of  the  antennae.  Following  the 
third  segment  of  the  brain  is  distinctly  seen  in  some  embryos 
a  short  segment  inserted  between  the  antennary  and  mandibular 
segments.  Its  short  ganglionic  swellings  {gl'^)  are  far  apart, 
and  connected  by  a  broad  commissure.  This  somite  may  also 
have  a  pair  of  small  invaginations,  but  I  have  been  unable  to 
find  them.  Hereupon  follow  the  mandibular,  and  the  first  and 
second  maxillary  segments,  each  with  a  pair  of  invaginations. 
Those  of  the  second  maxillary  segment  are  concealed  behind 
the  bases  of  the  elongated  appendages,  but  are  readily  seen  in 
sections. 

The  three  broad  thoracic  segments  are  provided  with  the 
three  pairs  of  legs,  all  of  which  are  of  the  same  length.  The 
division  into  femur,  tibia,  and  tarsus  is  indistinctly  marked.  In 
none  of  the  preceding  stages  have  I  observed  what  is  so  prom- 


354 


WHEELER. 


[VOL.  III. 


inent  in  Blatta,  viz.  the  appearance  of  appendages  on  the 
abdominal  somites.  There  are  not  the  slightest  traces  of  even 
the  pair  of  appendages  of  the  first  abdominal  somite,  which  in 
Blatta  develop  into  the  large  glandular  organ  of  which  I  have 
treated  elsewhere  (50).  The  tracheal  invaginations  are  situated 
at  the  bases  of  the  legs.  Those  of  the  first  thoracic  segment 
are  small,  and  soon  close  over  and  disappear.  The  second  pair, 
which  are  almond-shaped,  are  the  largest  in  the  whole  embryo, 
and  so  remain.  They  are  situated  near  the  constriction  divid¬ 
ing  the  first  from  the  second  thoracic  segment,  and  in  later 
stages  often  have  the  appearance  of  belonging  to  the  first  seg¬ 
ment.  The  metathoracic  invaginations  are  somewhat  smaller, 
and  are  also  placed  near  the  edges  of  the  somite  to  which  they 
belong.  In  the  succeeding  abdominal  segments  there  is  a 
tracheal  invagination  in  the  middle  of  each  lateral  half.  These 
invaginations  become  successively  smaller  till  they  can  be  de¬ 
tected  only  with  great  difficulty  on  the  loth  and  nth  somites 
{t  19,  t  20).  The  nth  somite  is  followed  by  the  broad  subhex- 
agonal  caudal  plate  with  its  large  protodaeal  invagination.  The 
corners  of  the  plate  are  formed  by  rounded  lobes  containing 
apparently  spherical  bodies,  the  ends  of  the  three  pairs  of  Mal¬ 
pighian  vessels.  These  grow  off  from  the  protodaeum  at  an 
unusually  early  period  in  Doryphora,  and  turn  back  till  their 
rounded  blind  ends  terminate  just  beneath  the  surface  ecto¬ 
derm.  The  paired  ganglionic  thickenings  are  seen  in  the 
embryo  figured  to  be  slightly  kidney-shaped  with  their  hili 
directed  laterally.  The  Mittelstrang  is  apparent  in  the  small 
and  nodular  intersegmental  thickenings  {insf),  which  appear 
from  the  surface  as  small  masses  of  cells  of  a  somewhat  dif¬ 
ferent  nature  from  those  of  other  portions  of  the  median  line. 

The  surface  changes  which  the  embryo  undergoes  in  the 
stages  immediately  following  that  represented  in  Fig.  72  may 
be  briefly  summarized.  The  embryo  just  described  lies  like  a 
band  on  the  ventral  yolk,  the  caudal  portion  still  extending 
round  the  hind  pole  of  the  egg,  and  up  the  dorsal  surface  a 
short  distance.  The  isochronous  changes  which  ensue  are,  (i) 
a  shortening  of  the  embryo,  bringing  the  tail  to  the  pole  of  the 
j  (2)  a  broadening  of  the  embryo,  the  sides  of  which  now 
bend  dorsally  and  clasp  the  yolk ;  (3)  a  greater  concentration 
of  the  cephalic,  mandibular,  and  maxillary  somites  to  form  the 


No.  2.] 


BLATTA  AND  DORYPHORA. 


35S 


head  of  the  larva,  and  (4)  an  increase  in  the  length  of  all  the 
appendages  except  the  antennae.  The  shortening  of  the  embryo 
stops  as  soon  as  its  tail  reaches  the  caudal  pole  of  the  egg,  the 
lateral  growth  of  the  body  continues,  and  we  reach  the  stage 
figured  (Fig.  74).  The  yolk  is  not  yet  covered  by  the  dorsad 
growing  walls  of  the  embryo.  The  head  is  distinctly  marked 
off  from  the  thorax,  the  wide-spread  mouth  parts  of  Fig.  72  have 
converged,  and  are  assuming  the  relations  which  they  bear  to 
one  another  in  the  larva.  The  ventral  nerve  chord  has  devel¬ 
oped  considerably,,  and  it  is  now  possible  to  recognize  near  the 
centre  of  each  ganglion  the  mass  of  ‘‘ Punktsubstanz"’  definitely 
marked  off  from  the  cellular  portion  and  united  with  the  corre¬ 
sponding  mass  of  its  fellow-ganglion  by  two-cross  commissures. 

Figure  75  represents  the  larva  ready  to  hatch.  The  dorsal 
body  wall  has  closed,  the  six  ocelli  have  become  pigmented, 
the  cuticle  has  developed  spines,  the  meso-  and  metathoracic 
and  first  abdominal  segments  have  each  developed  a  short, 
sharp,  black  spine  in  a  line  with  the  abdominal  spiracles. 
These  spines  are  used  by  the  larva  in  rupturing  the  chorion, 
the  vitelline  membrane,  and  the  various  cuticles  which  it  has 
shed  before  reaching  this  stage.  The  movements  of  the  hatch¬ 
ing  insect  at  first  produce  a  rent  in  the  chorion  extending  from 
the  first  to  the  third  spine,  by  further  struggling  the  two  rents 
from  opposite  sides  are  made  to  meet  over  the  head,  and  the 
insect  emerges  from  between  the  two  lips  thus  formed.  The 
embryonic  development  requires  about  six  days. 

Before  passing  on  to  a  description  of  the  internal  changes  of 
Doryphora  and  Blatta,  it  is  necessary  to  consider  the  fate  of  the 
embryonic  membranes  of  the  former  insect.  This  is  very  differ¬ 
ent  from  what  was  observed  in  Blatta.  The  serosa,  instead  of 
rupturing  when  the  amnion  ruptures,  separates  from  it  and  also 
from  the  entire  surface  of  the  yolk,  and  forms  a  third  egg  enve¬ 
lope,  beneath  the  vitelline  membrane,  to  which  it  applies  itself. 
It  remains  clearly  recognizable  by  its  large  and  deeply  staining 
nuclei  till  the  insect  is  almost  ready  to  hatch  (Fig.  86  sr)^  when 
it  disappears,  probably  by  absorption.  The  fate  of  the  amnion 
is  peculiar.  On  rupturing,  its  two  ventrally  bent  folds  turn  back 
and  become  in  part  applied  to  the  yolk.  A  few  of  its  cells  are 
loosened  from  the  bulk  of  the  membrane,  and  are  often  seen 
sticking  to  the  serosa  at  different  points.  They  are  probably 


356 


WHEELER. 


[VoL.  irr. 


endowed  with  amoeboid  tendencies,  for  when  the  ectodermic 
wall  is  about  to  be  completed  in  the  median  dorsal  line  these 
cells  are  seen  to  have  accumulated  at  the  place  of  closure.  The 
amniotic  cells,  which  have  become  applied  to  the  surface  of  the 
yolk  by  the  bending  back  of  the  two  folds  resulting  from  rup¬ 
ture,  have  closed  in  the  yolk  while  the  serosa  is  separating  from 
it.  The  advancing  body  walls  of  the  embryo,  however,  soon 
make  the  amniotic  covering  unnecessary,  and  it  contracts  in  the 
median  dorsal  line  to  form  what  may  be  called  an  amniotic  dorsal 
organ,  to  distinguish  it  from  the  serosal  dorsal  organ  of  Blatta. 
Figure  90  is  a  part  of  a  section  through  the  median  dorsal  por¬ 
tion  of  an  embryo  in  the  stage  represented  in  Fig.  74.  At  do 
the  protoplasm  of  the  amnion  has  thickened,  and  the  nuclei  are 
seen  passing  in  between  the  yolk  bodies.  At  m  are  a  number  of 
nuclei  undergoing  degeneration.  These  resemble  the  degener¬ 
ating  entoderm  nuclei  to  which  I  have  called  attention  in  a  much 
younger  stage.  The  amnion  cells,  which  have  become  applied 
to  the  yolk  when  the  membrane  ruptures,  enter  the  yolk  after 
the  formation  of  the  dorsal  organ  by  the  very  narrow  slit  left 
in  the  closing  ectoderm  in  the  median  dorsal  line.  This  is  seen 
somewhat  indistinctly  in  Fig.  85.  Nuclei  are  observed  at  b 
passing  in  between  the  two  cardioblasts  {cby  cb),  which  are  about 
to  meet  and  form  the  heart.  The  splanchnic  mesoderm  islni), 
with  the  underlying  entoderm,  still  leaves  a  wide  gap  through 
which  the  migration  into  the  yolk  takes  place.  The  granular 
matter  surrounding  the  nuclei  is  probably  the  remains  of  the 
cytoplasm  of  the  amnion  cells.  In  the  figure  a  number  of  entire 


Fig.  14. 


Fig.  16. 


Big  IS- 


Figures  14-16.  —  Three  diagrammatic  median  cross-sections  through  the  egg  of 
DoryphorUy  before  and  during  revolution,  ch.  chorion;  v.  vitelline  membrane,  ap¬ 
plied  tp  the  inner  face  of  the  chorion;  a.  amnion;  s.  serosa;  em.  embryo;  d.o.  dor¬ 
sal  organ  (amniotic)  ;  y.  yolk. 


No.  3.] 


BLATTA  AND  DORYPHORA. 


ssr 


amnion  cells  ici)  are  still  seen  just  beneath  the  serosa,  and  one 
is  seen  right  at  the  narrow  space  between  the  cardioblasts.  The 
last  steps  in  the  process  are  represented  in  Fig.  93.  The  dorsal 
ectoderm  has  become  continuous  at  ect  in  the  median  line.  The 
two  cardioblasts  are  still  in  the  same  stage.  One  of  the  last 
amnion  nuclei  is  passing  in  surrounded  by  a  mass  of  granules.. 
A  clear  idea  of  the  revolution  of  the  embryo  Doryphora  may  be 
obtained  from  the  three  stages  in  the  diagrams. 

The  method  of  revolution  just  described  is  very  similar  to  that 
observed  by  Graber  (15)  in  Lma.  Though  he  did  not  give  a 
description  of  the  complete  process,  he  made  the  important 
observation  that  the  serosa  remains  nnchanged  till  after  the  chiti- 
nous  cuticle  is  formed. 


General  Remarks. 

Dorsal  Organ. 

The  term  “dorsal  organ”  has  been  applied  to  the  peculiar 
thick  lump  of  cells  resulting  from  the  concentration  on  the  dor¬ 
sal  yolk  of  the  remains  of  either  the  amnion  or  serosa,  or  of  both, 
preparatory  to  their  sinking  into  the  yolk  and  being  absorbed. 

A  similarity  in  form  and  position  has  led  many  investigators 
to  look  for  an  homology  between  the  dorsal  organ  of  insects  and 
the  homonymous  organ  of  the  Crustacea. 

My  observations  on  the  dorsal  organ  of  Cymothoa  have  con¬ 
vinced  me  that  there  are  fundamental  differences  between  the 
Crustacean  and  Hexapod  dorsal  organ.  First,  the  dorsal  organ 
of  this  form,  and  probably  other  Isopoda,  is  a  structure  which 
persists  from  an  early  stage  almost  to  hatching,  and  may  persist 
throughout  life  in  some  Branchiopoda,  whereas  the  dorsal  organ 
of  insects  is  a  very  transitory  structure.  Secondly,  the  dorsal 
organ  of  Cymothoa  seems  to  be  a  secretory  organ,  as  was  deemed 
probable  by  Balfour  (2).  I  have  observed  that  the  elongated 
cells  which  form  the  organ  secrete  a  reniform  sack  of  chitin, 
which  is  joined  by  means  of  a  corrugated  chitinous  tube  to  the 
cuticle  shed  from  the  surface  of  the  embryo  at  a  very  early 
stage.  Nusbaum  (34)  has  observed  that  the  cavities  of  the 
dorsal  organs  of  My  sis  are  filled  with  a  clear  substance,  probably 
a  secretion. 

The  presence  of  the  so-called  dorsal  organ  in  insects  is  prob- 


35^ 


WHEELER. 


[VOL.  III. 


ably  due  to  the  fact  that  the  embryonic  envelopes  are  to  be 
absorbed.  As  these  membranes  consist  of  assimilable  matter, 
it  is  obviously  an  advantage  to  the  embryo  to  be  able  to  add 
them  to  the  stock  of  food  represented  by  the  yolk.  The  simplest 
conceivable  method  of  effecting  the  resolution  of  the  envelopes 
into  food  material,  considering  their  position  when  fully  devel¬ 
oped,  would,  of  course,  be  to  engulf  them  in  the  yolk,  where, 
under  the  influence  of  the  yolk  cells,  metabolism  is  being 
actively  carried  on.  There  are  two  methods  of  inclosing  the 
membranes  in  the  yolk.  According  to  one,  they  might  undergo 
dissolution  in  siUi ;  according  to  the  other,  they  might  be 
brought  together  in  a  mass  and  swallowed  up  by  the  yolk  some¬ 
where  in  the  median  dorsal  line.  Obviously  the  latter  method 
is  the  more  advantageous,  as  the  body  walls,  continually  grow¬ 
ing  towards  the  median  dorsal  line,  might  be  impeded  in  their 
advance  if  the  membranes  were  absorbed  at  all  points  on  the 
surface  of  the  yolk.  Probably  the  inconvenience  which  would 
thus  result  from  a  diffuse  absorption  accounts  for  the  fact  that 
it  does  not  occur,  though  a  modification  of  the  method  occurs 
in  Doryphora,  where  the  serosa  is  absorbed  very  late  in  develop¬ 
ment  after  the  larva  has  secreted  its  second  cuticle  and  is  almost 
ready  to  leave  the  egg. 

Given  a  thickening,  somewhat  flattened  mass  of  cells,  des¬ 
tined  to  be  swallowed  up  in  the  yolk,  and  it  is  most  natural  to 
suppose  that  the  organ,  in  passing  into  the  yolk,  would  become 
cup-shaped  as  in  Blatta,  or  form  a  thick-walled  tube,  if  the  organ 
extended  the  full  length  of  the  dorsum,  as  in  Hydrophilns.  In 
either  case  the  outer  ends  would  be  made  to  converge,  by  the 
lateral  pressure  of  the  yolk  and  the  sinking  of  the  median  por¬ 
tion  of  the  organ,  and  we  should  get  a  closed  tube  or  sack.  This 
would  not,  of  course,  hold  true  of  an  organ  formed  like  the 
amniotic  dorsal  organ  of  Doryphora^  for  the  reason  that  in  this 
case  the  decomposition  begins  as  soon  as  the  organ  is  formed, 
and  not  after  it  has  passed  into  the  yolk,  as  is  the  case  with 
the  serosal  dorsal  organs  of  other  forms  {Hydrophilns ,  BlattOy 
Neophalax).  Hence  the  cavity  of  the  Hexapod  dorsal  organ 
would  resemble  the  cavity  or  so-called  micropyle  of  the  Isopod 
dorsal  organ,  though  the  two  cavities  would  not  be  homologous. 


No.  2.] 


BLATTA  AND  DORYPHORA. 


359 


The  Fate  of  the  Different  Germ  Layers. 

A.  Entoderm. 

Recent  writers  on  insect  embryology  have  recognized  two 
forms  of  entoderm,  —  one  called  primary,  and  represented  by 
the  yolk  cells ;  and  one  secondary,  represented  by  the  epithe¬ 
lial  wall  of  the  mesenteron.  In  Blatta  the  yolk  nuclei  steadily 
increase  in  size  from  their  first  appearance  at  a  time  when  they 
are  no  larger  than  other  nuclei.  At  the  time  of  the  formation 
of  the  dorsal  organ  they  are  by  far  the  largest  nuclei  in  the  egg 
(Fig.  v).  Their  chromatin  is  distributed  through  the  karyo- 
plasm  in  the  form  of  a  fine,  convoluted  thread,  and  as  two 
or  more  nucleoli  (Fig.  54  v).  Soon  after  the  closing-in  of  the 
yolk,  they  lose  their  rounded  outline,  and  become  irregular  and 
more  homogeneous  (Fig.  55  v).  In  the  last  stages  of  their 
dissolution  they  may  be  seen  as  stellate  spots  in  the  remains  of 
the  yolk  aggregated  in  the  stomach  of  the  advanced  embryo 
(Fig.  48).  Yolk  segmentation,  though  occurring  in  Blatta,  takes 
place  after  the  appendages  are  formed,  at  a  much  later  period 
than  in  DorypJiora.  The  segments,  usually  very  obscurely  de¬ 
fined,  become  confluent  again  as  development  continues. 

In  DorypJiora  the  yoke  cells  undergo  no  increase  in  size  from 
the  time  of  their  first  appearance ;  but  soon  after  the  yolk  has 
become  segmented  their  cytoplasm  is  reduced  to  a  scarcely 
perceptible  layer  surrounding  the  nucleus,  which  has  become 
irregularly  polygonal  (Fig.  83).  As  all  the  eggs  I  studied  were 
killed  and  preserved  in  the  same  manner,  this  difference  in 
form  between  the  yolk  nuclei  in  the  stages  during  yolk  seg¬ 
mentation  and  after  this  process  till  the  setting-in  of  degenera¬ 
tion,  must  be  regarded  either  as  a  normal  change  in  the  living 
nuclei,  or  as  indicating  that  their  chemical  nature  is  changed, 
and  their  resistance  to  the  altering  effects  of  reagents  lessened. 

After  the  completion  of  the  mesenteron  at  a  time  when  the 
larva  is  almost  ready  to  hatch,  the  remains  of  the  yolk  nuclei 
are  pushed  back  into  the  stomach  in  the  same  way  as  in  Blatta. 
Here  they  degenerate  in  a  manner  very  similar  to  that  observed 
in  the  secondary  entoderm  nuclei,  and  the  nuclei  of  the  amniotic 
dorsal  organ.  They  become  swollen  and  vesicular,  and  their 
chromatin  is  reduced  to  irregular  masses.  The  yolk  becomes  a 
compact,  granular  mass,  staining  pink  in  borax  carmine. 


36o 


WHEELER. 


[VOL.  III. 


The  origin  of  the  secondary  entoderm  in  Doryphora  has  been 
treated  of  at  length  in  preceding  paragraphs.  We  have  now  to 
trace  its  fate,  from  the  condition  in  which  we  left  it,  as  two 
masses,  —  one  under  the  blind  end  of  the  oesophagus,  and  the 
other  under  the  blind  end  of  the  proctodaeum.  In  an  embryo 
of  the  stage  of  Fig.  72,  each  of  the  isolated  masses  has  begun 
to  send  out  the  two  bands  of  entoderm.  These  diverge  from 
their  point  of  origin,  and  apply  themselves  to  what  is  to  be  the 
splanchnic  mesoderm  (Fig.  78  ent).  Their  divarication  is  so 
slight  that  a  thick  longitudinal  section  will  sometimes  include 
a  whole  band,  if  still  short.  Thus,  in  Fig.  92,  from  a  section 
through  an  embryo  in  the  stage  of  Fig.  72  ;  passed  to  one  side 
of  the  median  line,  we  have  the  entoderm  {ent)  still  attached  to 
the  mesoderm  {msd),  which  is  now  distinctly  marked  off  from 
the  ectoderm  of  the  stomodaeal  invagination  {st).  The  entoderm 
is  attached  to  the  inner  end  of  the  oesophagus,  and  extends 
along  the  yolk  as  a  band  two  or  three  cells  thick.  It  is  clearly 
distinguishable  from  the  mesoderm  by  the  greater  clearness  of 
its  cells,  and  by  its  paler  nuclei.  The  process  at  the  posterior 
end  of  the  embryo  is  similar,  as  may  be  concluded  from  Fig. 
76,  from  the  same  embryo.  Like  the  anterior  thickening,  the 
posterior  mass  does  not  remain  stationary,  but  grows  out  in  two 
bands.  Thus  it  happens  that  little  entoderm  is  found  right 
under  the  proctodaeum.  The  knife  has  not  passed  through  the 
proctodaeal  invagination  in  the  figure,  but  has  cut  to  one  side 
of  the  median  line,  through  the  Malpighian  vessels  mp^  and 
7np^  of  Fig.  72.  The  true  relations  of  these  vessels  may  be 
understood  from  an  examination  of  Fig.  77,  and  the  present 
section.  Fig.  76.  The  strand  of  entoderm,  very  similar  to  the 
anterior  strand  described  above,  is  attached  to  the  proctodaeal 
pocket  near  the  point  at  which  one  of  the  first  pair  of  Mal¬ 
pighian  tubes  {mpg^)  turns  out  toward  the  surface  of  the  embryo, 
pushing  aside  the  mesoderm,  which  elsewhere  forms  a  continuous 
sheet  under  the  ectoderm. 

In  the  same  figure  may  be  noticed  a  second  mass  of  entoderm 
{ent)  attached  to  the  proctodaeum  near  the  point  at  which  the 
third  pair  of  Malpighian  tubes  branches  off  from  the  common 
proctodaeal  pocket.  This  mass  ends  with  a  sharp  point  between 
two  yolk  segments.  I  have  seen  similar  masses  of  entoderm  in 
a  few  other  embryos,  apparently  independent  of  the  two  main 


No.  2.] 


BLATTA  AND  DORYPHORA. 


361 

forward-growing  strands,  and  terminating  in  the  same  acute 
point  which  seems  to  force  its  way  between  the  yolk  segments. 
These  points  of  entoderm  do  not  grow  far,  as  I  have  concluded 
from  an  examination  of  slightly  older  stages,  but  soon  fuse  with 
the  bases  of  the  two  main  strands,  and  form  a  meniscoid  mass 
in  every  way  comparable  with  the  watch-glass-shaped  mass  in 
Miisca,  as  described  by  Kowalevsky  (27). 

A  median  cross-section  of  an  embryo,  with  the  entoderm 
bands  fully  established,  but  not  confluent,  is  shown  in  Fig.  78. 
The  embryo  is  cut  in  two  places.  The  upper  half  passes  through 
one  of  the  basal  abdominal  segments,  while  the  lower  half  passes 
through  the  abdomen,  a  short  distance  from  the  tail.  In  the 
upper  half  no  entoderm  cells  are  to  be  found,  as  the  two  bands 
have  not  yet  reached  in  their  forward  growth  the  basal  abdominal 
segments  in  which  they  fuse  with  the  two  bands  growing  back 
from  the  stomodeeal  invagination.  In  the  lower  half  a  heap  of 
succulent  entoderm  cells  is  seen  on  each  side,  separated  from 
the  coelomic  cavity  (cl)  by  the  splanchnic  mesoderm  {slm). 

Four  stages  in  the  formation  of  the  mesenteron  after  the 
establishment  of  the  entoderm  as  two  long  bands,  are  repre¬ 
sented  in  Figs.  83  to  86  e7tt.  The  entoderm  remains  throughout 
embryonic  development  perfectly  distinct  from  the  splanchnic 
mesoderm,  to  which  it  is  nevertheless  very  closely  applied.  At 
first  the  cells  are  irregularly  arranged  in  the  band  which  is 
deepest  in  the  middle,  but  gradually  flattens  out  to  a  single  cell 
in  thickness  at  its  dorsal  and  ventral  edges  (Fig.  83).  In  a  later 
stage,  however,  the  nuclei  have  their  long  axes  directed  at  right 
angles  to  the  long  axes  of  the  splanchnic  mesoderm  cells  (Fig. 
84),  and  thus  indicate  that  the  cells  of  the  entoderm  are  begin¬ 
ning  to  assume  a  definite  columnar  arrangement,  though  they 
still  lie,  in  some  places,  in  two  or  more  rows,  one  above  the 
other.  By  the  time  the  body  walls  are  about  to  close,  the  cells 
of  the  entoderm  have  formed  an  even  layer  of  columnar  ele¬ 
ments  (Fig.  85),  an  arrangement  which  is  retained  in  all  the 
subsequent  stages  till  the  embryo  hatches. 

The  growth  of  the  entoderm,  accompanying  the  adjacent 
mesoderm  and  ectoderm  in  their  dorsad  movement,  is  at  first 
largely  along  the  dorsal  edges  of  the  bands,  as  may  be  seen  by 
comparing  Figs.  84  and  85,  where  the  distance  between  the 
ventral  edges  of  the  two  bands  is  nearly  the  same,  while  the 


362 


WHEELER. 


[VoL.  IIL 


distance  between  the  dorsal  edges  is  greatly  lessened.  The 
transformation  of  the  original  ribbon  of  several  superimposed 
rows  of  cells  into  the  simple  epithelium  of  columnar  cells  is  not 
entirely  due  to  cell  division.  As  may  be  seen  from  Figs.  84 
and  85,  either  the  wandering  of  the  inner  rows  of  cells  over  the 
outer  towards  the  dorsal  edge  of  the  ribbon,  or  a  stretching  of 
the  whole  band,  so  as  to  permit  an  intercalation  of  the  cells 
of  the  inner  rows  between  those  of  the  outer  row,  are  the  more 
probable  factors  in  the  thinning  out  of  the  entoderm.  The 
latter  method  is  more  probable,  though  the  former  method  is 
certainly  in  keeping  with  the  gliding  and  mobile  movements  of 
the  entoderm.  The  nuclei  of  the  entoderm  have  their  chromatin 
distributed  in  the  typical  filament,  which  is  more  attenuated 
than  in  either  mesoderm  or  ectoderm  nuclei.  Shortly  before 
hatching  the  chromatin  of  the  mesenteron  nuclei  appears  to 
have  dissolved,  as  they  seem  to  have  become  perfectly  homo¬ 
geneous,  though  they  still  stain  deeply.  In  the  hatching  larva 
the  cells  have  become  more  deeply  columnar,  on  account  of  a 
diminution  in  calibre  of  the  mesenteron.  The  nuclei  cease  to 
absorb  more  of  the  staining  fluid  from  the  surrounding  eyto- 
plasm,  though  the  walls  retain  their  evenly  rounded  contour. 
Such  a  fundamental  change  in  the  nuclei  would  seem  to  indicate 
that  some  important  change  is  about  to  take  place  in  the  mes¬ 
enteric  layer  of  cells  ;  but  whether  this  change  is  dissolution  I 
am  unable  to  say,  as  I  have  not  studied  the  insect  in  the  stages 
beyond  hatching. 

The  process  of  mesenteron  formation  is  essentially  the  same 
in  Blatta  as  that  just  described  for  Doryphora.  From  the  first, 
the  entoderm  cells  of  Blatta  are  as  small  and  indistinct  as  the 
yolk  cells  are  large  and  prominent.  They  form,  as  stated  above, 
two  bands  of  very  flat  cells  bearing  the  same  relations  to  the 
mesoderm  as  the  corresponding  bands  of  Doryphora  (Fig.  54 
ent).  The  edges  of  the  two  ribbons  continue  their  growth,  and 
meet  ventrally  and  dorsally,  to  complete  the  mesenteron  (Fig. 

55 

Besides  the  lining  of  the  mesenteron  the  corpus  adiposajn^ 
represented  during  the  embryonic  life  of  Doryphora  by  a  number 
of  granular  cells  which  constantly  increase  in  size  up  to  the  time 
of  hatching,  probably  originates  from  the  entoderm.  I  have 
observed  in  several  cases  that  before  the  two  posterior  bands  of 


No.  2.] 


BLATTA  AATD  DORYPHORA. 


363 


entoderm  have  reached  the  middle  of  the  embryo  a  number  of 
granular  and  somewhat  larger  cells  are  to  be  found  mingled  with 
the  cells  of  the  bands.  I  conclude  that  these  cells  are  of  ento- 
dermic  origin  because  when  first  seen  they  are  associated  with 
the  entoderm  cells  and  resemble  them  more  closely  than  they 
resemble  the  adjacent  mesodermic  elements.  At  first  small 
(Fig.  85  ad)^  these  fat  cells  gradually  but  constantly  increase  in 
size,  their  cytoplasm  and  nuclei  increasing  in  about  the  same 
ratio.  They  wander  about  in  the  body  cavity,  but  finally  attach 
themselves  to  the  ectodermic  body  walls,  especially  in  the  pos¬ 
terior  two-thirds  of  the  embryo  on  each  side  of  the  heart  (Fig. 
86  ad).  They  remain  more  or  less  globular  or  oval,  the  side  in 
contact  with  the  wall  hollowing  out  a  concavity  in  the  cells  of 
the  ectoderm.  The  granulation  of  the  cytoplasm  which  first 
distinguishes  the  fat  cells  from  the  true  entodermic  elements 
becomes  coarser  with  the  increase  in  volume.  In  the  embryo 
ready  to  hatch  the  adipose  cells  have  acquired  gigantic  dimen¬ 
sions,  being  many  times  the  size  of  those  represented  in  Fig.  86. 
Both  nuclei  and  cytoplasm  stain  deeply,  so  that  these  fat  cells 
are  rendered  among  the  most  conspicuous  objects  in  a  section. 

B.  Mesoderm. 

In  Doryphora  as  soon  as  the  gastrular  tube  has  collapsed,  the 
polygonal  mesodermic  elements  form  a  layer  several  cells  in 
thickness,  applied  to  the  inner  surface  of  the  median  ventral 
ectoderm  (Fig.  65  msd).  This  layer  of  cells  thins  out  at  its 
lateral  edges.  With  the  first  traces  of  segmentation  in  the  outer 
layer  the  mesodermic  layer  also  divides,  though  incompletely,  at 
the  same  places  of  constriction  (Fig.  82).  Soon  the  single 
intersegmentally  divided  band  of  mesoderm  splits  in  the  median 
line  so  that  each  segment  contains  two  subquadrangular  flat¬ 
tened  masses.  The  coelomic  cavity  is  formed  at  the  outer 
edge  of  each  of  the  masses  by  a  separation  of  the  cells  of  the 
two  layers  (Fig.  78  cl).  The  inner  constitutes  the  splanchnic 
mesoderm,  while  all  the  remainder  of  the  mesoderm  constitutes 
the  somatic  layer  (Fig.  78  shn).  When  the  appendages  appear, 
it  is  the  latter  layer  of  cells  which  supplies  their  cavities  with 
muscle-forming  cells ;  the  portions  inclosing  the  coelomic  cavity 
accompany  the  adjacent  ectoderm  in  its  dorsad  growth.  As 


3^4 


WHEELER. 


[vbL.  rrr. 


soon  as  the  growth  in  this  direction  is  fairly  started  the  ecto¬ 
derm  bulges  out  (Fig.  83  ect)  and,  drawing  with  it  the  somatic 
mesoderm,  leaves  a  cavity  between  the  yolk  and  the  embryo 
which  soon  communicates  with  the  coelomic  cavity  and  assumes 
large  dimensions  (Fig.  84).  Through  the  body  cavity  thus 
formed  a  thin  plasma  found  as  coagulated  masses  in  hardened 
embryos  probably  circulates. 

The  cell  cb  (Fig.  83),  which  is  recognized  even  at  a  much 
earlier  stage  by  its  peculiar  form,  and  which  is  destined  to  take 
part  in  the  formation  of  the  heart,  is  the  only  element  still 
uniting  the  splanchnic  and  somatic  layers.  This  cell,  as  may  be 
clearly  seen  in  Figs.  83  and  84  cb^  is  triangular  in  cross-section, 
and  inserts  one  of  its  acute  angles  between  the  yolk  and  the 
ectoderm.  As  this  cell  with  those  of  exactly  the  same  shape 
anterior  and  posterior  to  it  form  the  heaft,  I  shall  call  it  a  cardio- 
blast.  The  true  shape  of  the  cardioblasts  may  be  seen  in  a  thick 
frontal  section  (Fig.  89)  through  the  embryo  of  which  Fig.  84  is 
a  cross-section.  Here  the  compact  row  of  crowded  but  regular 
heart -forming  elements  {cb)  is  seen  running  between  the  actively 
proliferating  entoderm  cells  {e7it)  on  the  one  hand,  and  the  meso¬ 
derm  cells  {msd)  on  the  other.  These  last  are  somewhat  scat¬ 
tered  in  the  space  between  the  cardioblasts  and  the  thickened 
ectoderm. 

In  the  more  advanced  embryo  (Fig.  85)  the  cardioblasts  {cb) 
from  either  side  are  near  together.  They  have  retained  their 
characteristic  form  and  position,  while  the  somatic  mesoderm 
has  been  converted  into  muscles  {^nsl)  and  connective  tissue, 
and  the  splanchnic  layer  {shn)  has  applied  itself  closely  to  and 
is  coextensive  with  the  single-celled  layer  of  entoderm  {ent). 
In  Fig.  86  the  heart  is  completed.  A  glance  at  this  figure  and 
Fig.  85  shows  the  manner  in  which  the  two  cells  from  opposite 
sides  unite.  Though  forming  the  two  halves  of  the  tube  in 
Fig.  86,  they  still  show  the  three  angles  which  were  apparent 
just  after  the  formation  of  the  coelomic  cavity.  The  heart 
remains  in  the  condition  shown  in  Fig.  86  till  the  embryo 
hatches.  I  have  not  studied  the  formation  of  the  blood  in 
Doryphora. 

My  observation  on  the  formation  of  the  sexual  organs,  though 
more  complete  than  in  Blatta,  are  still  very  fragmentary.  These 
organs  originate  as  two  elongate  thickenings  of  splanchnic 


No.  2.J 


B'LATTA  AND  DORYPHORA. 


mesoderm,  one  on  each  side  projecting  into  the  body  cavity. 
Later  (Fig.  84  they  become  rounded  and  are  attached  by  a 
thin  band  of  splanchnic  mesoderm  only.  I  have  seen  the  much 
attenuated  duct  leading  from  each  organ  to  the  exterior,  but 
have  made  no  observations  on  its  origin.  The  ducts  converge 
posteriorly,  but  end  by  separate  openings  on  the  i  ith  abdominal 
somite ;  thus  presenting  a  condition  which  in  Ephemerids  is 
permanent  throughout  life,  according  to  Palmen  (37).  There 
is  probably  some  connection  between  the  two  pairs  of  very 
indistinct  tracheal  openings  in  the  lOth  and  nth  somites  and 
the  openings  of  the  efferent  ducts,  but  I  was  unable  to  deter¬ 
mine  whether  the  large  sexual  openings  originate  by  enlarge¬ 
ment  from  a  single  pair  of  these  tracheal  openings,  or  from  the 
confluence  of  all  four  to  form  two  orifices.  The  cross-section 
(Fig.  80)  includes  the  openings  of  the  efferent  ducts  {go  go),  the 
knife  having  taken  away  a  very  thin  layer  of  surface  cells. 

Up  to  the  formation  of  the  coelomic  cavities  the  mesoderm  of 
Blatta  closely  resembles  the  same  layer  in  Doryphora.  With 
the  evagination  of  the  appendages  from  the  entoderm  a  decided 
difference  is,  however,  observable.  Each  coelomic  segment,  if 
situated  in  an  appendage-bearing  segment,  instead  of  retreating 
dorsally  as  in  Doryphora,  sends  a  diverticulum  into  the  appen¬ 
dage.  This  is  clearly  seen  in  Fig.  53  cl,  a  cross-section  from  an 
embryo  twelve  days  old.  The  cells  of  the  diverticulum  develop 
into  the  muscles  of  the  appendage,  and  together  with  a  portion  of 
the  mesodermic  layer  still  remaining  in  the  body  cavity  are  shut 
off  from  what  probably  represents  the  true  coelomic  cavity  (Fig. 
54  cl).  The  further  changes  again  resemble  those  in  Doryphora. 
Long  before  the  heart  is  formed  and  the  lateral  walls  have  met 
,  in  the  median  line,  the  body  walls  of  the  embryo  are  observed 
to  pulsate  regularly  like  the  body  walls  of  Gryllotalpa,  as 
described  by  Korotneff  (25).  As  in  Doryphora  a  plasma  is  at 
this  time  found  in  the  body  cavity  which  is  divided  by  films  of 
connective  tissue  into  a  great  number  of  small  intercommunicat¬ 
ing  lacunae  (Fig.  54).  In  regard  to  the  primitive  blood  sinus, 
my  observations  confirm  Patten’s  (38).  He  says  :  “The  primitive 
blood  sinus  is  the  space  between  the  somatic  and  splanchnic 
mesoderm,  divided  into  a  number  of  smaller  and  irregular 
sinuses  by  meshes  of  connective  tissue,  some  cells  of  which,  in 
the  earlier  stages,  become  free  and  form  the  blood  capsules.  By 


WHEELER. 


[VOL.  III. 


366 

the  pulsation  of  the  mesodermic  folds,  long  before  a  special 
heart  is  formed,  a  circulation  through  the  body  cavity  is 
brought  about  like  the  circulation  in  many  of  the  lower  worms.” 

One  of  the  stages  in  the  formation  of  the  heart  is  seen  in  Fig. 
52.  The  cardioblasts  {cb  cb)  are  both  more  numerous  and  more 
irregular  than  in  Doryphora.  They  unite  to  form  a  tube,  the 
lumen  of  which  is  at  first  oblong  in  cross-section  (Fig.  52  Ji). 
In  the  cardiac  walls  amoeboid  cells  are  occasionally  seen  ibl), 
which  loosen  themselves  from  the  mesodermic  elements  and 
pass  into  the  lumen  of  the  tube,  probably  to  form  blood  cor¬ 
puscles. 

C.  Ectoderm. 

My  observations  on  the  organs  derived  from  the  ectoderm  are 
limited  almost  exclusively  to  Do7ypho7'a. 

The  hypodermis  of  the  embryo  secretes  two  cuticles,  the 
second  of  which  covers  the  larva,  while  the  first  is  cast  off  at 
the  time  of  hatching.  Shortly  before  hatching  the  embryo  is 
confined  by  four  loose  envelopes,  —  the  chorion,  the  vitelline 
membrane,  the  serosa,  and  the  first  cutiele.  Graber  (15)  has 
made  a  similar  observation  on  the  embryo  Lina. 

The  three  broad-based  chitinous  spines  used  by  the  insect  in 
rupturing  its  envelopes,  and  which  are  analogous  to  the  frontal 
spine  observed  in  Strojigylosoma  by  Metschnikow  (31)  and  the 
deciduous  claw  on  the  beaks  of  birds,  are  secreted  by  pyramidal  ' 
thickenings  of  the  hypodermis  (Fig.  86  hsp),  the  cells  of  which 
are  much  lengthened,  though  forming  a  single  layer. 

In  the  surface  views  of  the  embryo  (Fig.  72)  it  is  possible  to 
trace  the  origin  of  all  the  ganglia  as  paired  thickenings  of  the 
outer  layer.  At  first  these  thickenings  differ  histologically  in 
no  particular  from  the  surrounding  ectoderm  (Fig.  78  ;/r). 
Gradually,  however,  the  cells  in  the  centre  of  each  thickening 
enlarge,  while  their  cytoplasm  becomes  drawn  out  into  fine 
threads.  At  the  same  time  all  the  ganglion  cells  thus  formed 
arrange  themselves  in  such  a  way  as  to  have  their  threads  inter¬ 
mingle.  This  mass  of  hiteiLzvined  threads  becomes  the  P2inktsub- 
stanz  (Fig.  94  to  104  pet).  The  outer  layer  of  cells  if)  contin¬ 
uous  with  the  hypodermis  {ecd)  stands  off  somewhat  from  the 
ganglionic  thickenings,  leaving  a  space  which  is  in  early  stages 
occupied  by  several  large,  clear,  oval  cells  (gbl),  which  divide 


No.  2.] 


BLATTA  AND  DORYPHORA. 


367 


rapidly  by  karyokinesis,  and  might  be  called £‘ang’/wd/asts,  as  the 
products  of  their  divisions  reinforce  the  mass  of  ganglion  cells. 
In  a  series  of  sections  through  the  mesothoracic  pair  of  ganglia 
(Figs.  94  to  104)  the  Mittlestrang  may  be  readily  traced.  The 
shape  of  its  cross-section  varies  with  the  plane  of  section 
through  the  ganglion  and  its  cross  or  longitudinal  commis¬ 
sures.  At  the  two  points  in  the  ganglion  where  the  two  pairs 
of  Punktsubstanz  masses  fuse  to  form  the  commissures  c  cnty  the 
Mittlestrang  is  in  great  part  obliterated.  The  median  strand  is 
largest  where  the  two  longitudinal  commissures  are  passing  into 
the  anterior  ends  of  the  three  thoracic  ganglia.  Here  they 
persist  in  the  larva,  while  completely  disappearing  elsewhere, 
and  become  converted  into  the  three  chitinons  fiircce,  each  one  of 
which  is  just  in  front  of  a  thoracic  ganglioji. 

Figure  105,  from  a  section  through  the  fore  end  of  the  meta- 
thoracic  ganglion,  shows  the  Mittlestrang  portion  (inst)  contin¬ 
uous  with  the  hypodermis  {ect)  and  broadening  out  into  the 
furca  (/)  after  passing  between  the  two  halves  of  the  ganglion 
{gl).  Muscles  {7?tsl)  are  attached  to  the  two  divergent  ends. 
The  mesothoracic  furca,  which  is  formed  in  exactly  the  same 
manner,  is  seen  in  Fig.  86  f,  where  it  passes  between  the 
commissures  {cm)',  its  connection  with  the  muscles  is  seen 
at  r. 

A  frontal  section  (Fig.  91)  shows  the  structure  of  the  meso- 
and  meta-thoracic  ganglia  after  they  have  become  loosened  from 
the  surface  ectoderm.  The  longitudinal  {cm)  and  cross-com¬ 
missures  {c  cm)  are  clearly  seen  as  white  Punktsubstanz  sepa¬ 
rated  from  the  ganglionic  cells  by  the  inner  neurilemma  {inrl). 
The  outer  neurilemma  {onrl)  is  also  developed,  as  are  also  the 
two  main  nerve  trunks  {fd-,  ft),  the  anterior  of  which  bifurcates 
{n)  while  leaving  the  ganglion. 

The  separation  of  the  nervous  system  from  the  integumen¬ 
tary  ectoderm  progresses  from  before  backwards.  The  two 
brain  masses  separate  first.  The  first  segment  becomes  very 
small  and  possibly  disappears.  The  three  segments  of  the 
optic  ganglion  are  invaginated  and  pushed  under  the  optic 
plate  in  a  manner  which  I  believe  to  be  similar  to  that  de¬ 
scribed  by  Patten  (39)  in  Acilius,  though  I  have  not  followed 
the  details  of  the  process.  The  frontal  ganglion  is  formed  as  an 
unpaired  thickening  of  the  dorsal  imll  of  the  oesophageal  ecta- 


368 


WHEELER. 


[VOL.  III. 


derm  Jiear  the  base  of  the  labriim.  Punktsubstanz  is  formed  in 
this  ganglion  in  the  same  manner  as  in  the  brain  and  ventral 
ganglia.  It  is  ultimately  loosened  from  the  stomodaeum,  and 
becomes  surrounded  by  mesodermic  elements.  Later  the  four 
pairs  of  ganglia  of  the  intercalary,  mandibular,  ist  and  2d  max¬ 
illary  segments  fuse  and  form  the  infraoesophageal  ganglion. 

That  the  outer  neurilemma  is  of  ectodermic  and  not  of  meso¬ 
dermic  origin  seems  to  be  proved  by  the  fact  that  shortly  after  the 
separation  of  the  nerve-chord  from  the  integnmentary  ectoderm^  it 
sheds  from  its  surface  a  delicate  chitinous  cuticle  simultaneotisly 
zvith  the  shedding  of  the  first  integumentary  ciUicle,  This  cuticle, 
which  is  separated  from  the  surface  of  the  outer  neurilemma,  and 
even  from  the  surfaces  of  the  main  neural  trunks,  is  afterwards 
absorbed. 

The  six  ocelli  are  formed  as  apple-shaped  thickenings  of  the 
optic  plate.  Their  small  size  has  hindered  me  from  studying 
their  structure  in  detail.  I  have  represented  in  Fig.  8i,  soon 
after  their  first  appearance,  two  of  the  ocelli  corresponding  to 
the  eyes  of  Acilius  numbered  V.  and  VI.  by  Patten.  Each 
forms  a  slight  depression  somewhat  paler  than  the  surrounding 
ectoderm.  The  nucleus  {ft)  of  one  of  the  central  cells  is  seen 
to  be  much  larger  than  the  nuclei  of  the  surrounding  cells. 
Patten  has  described  and  figured  this  same  large  nucleus  in  the 
eyes  of  Acilius  (PI.  XL,  Figs.  63,  64,  65,  etc.). 

The  five  pairs  of  invaginations  anterior  to  those  of  the  second 
maxillary  segment  form  the  tentorium  of  the  larval  head.  These 
invaginations  grow  inwards  as  slender  tubes,  which  anastomose 
in  some  places.  Their  lumina-  are  ultimately  filled  with  chitin. 
Palmen  (36)  found  that  the  tentorium  of  Ephemerids  breaks 
across  the  middle  during  eedysis,  and  that  each  half  is  drawn 
out  of  the  head,  like  the  chitinous  lining  of  a  tracheal  tube. 
This  fact,  together  with  my  observations  on  the  tentorium  of 
Doryphora,  makes  it  highly  probable  that,  as  Palmen  suggests, 
the  tentorium  is  formed  from  tracheae,  which  have  become 
modified  for  muscular  attachment. 

Of  the  true  tracheal  invaginations,  those  of  the  pro-  and 
meta-thorax  disappear.  The  mesothoracic  spiracle  comes  to  lie 
between  the  pro-  and  meso-thoracic  segments  near  the  base  of 
the  legs  and  ventral  to  the  line  of  abdominal  spiracles.  This 
first  spiracle  is  the  largest,  and  in  cross-section  appears  as  a 


No.  2.] 


BLATTA  AND  DORYPHORA. 


369 


short,  chitinous  cylinder,  projecting  somewhat  from  the  general 
surface  of  the  segment.  Its  inner  walls  are  lined  with  spines 
which  are  directed  outward.  The  abdominal  spiracles,  though 
smaller,  are  also  lined  with  similar  spines  (Fig.  85  tr).  The  first 
pair  of  tracheae  send  large  branches  to  the  head.  The  abdominal 
tracheae  of  each  side  of  the  body  anastomose  to  form  a  longi¬ 
tudinal  trunk,  from  which  the  branches  ramify  to  the  different 
organs.  The  invaginated  ectodern,  at  first  very  thick  (Fig.  84 
tr)y  gradually  thins  out  as  the  respiratory  tubes  lengthen  and 
ramify.  The  thin  epithelium  thus  formed  secretes  the  chitinous 
lining  provided  at  the  time  of  hatching  with  the  spiral  thicken¬ 
ings  so  characteristic  of  insect  tracheae. 

The  proctodaeum  and  stomodaeum  when  first  formed  are  tri¬ 
angular  in  cross-section.  Later  both  become  hexagonal  (Figs. 
79  and  80).  A  like  pronounced  similarity  in  form  between  the 
stomodaeum  and  proctodaeum  of  Gammariis  has  been  observed 
by  Pereyaslawzewa  (40).  She  says  :  “Fait  tres  interessant,  qui 
merite  d’etre  note,  c’est  qu’a  mesure  de  I’acroissement  du  rectum 
et  de  I’oesophage,  leur  partie  interieure  affecte  absolument  la 
meme  forme  carree,  dont  les  parois  sont  concaves.  Ce  qui 
concerne  la  configuration  des  cavites,  elles  n’en  different  aucune- 
ment  et  se  dessinent  sous  forme  d’une  croix  oblique ;  la  dis¬ 
semblance  consiste  en  ce  que  dans  I’oesophange  ce  sont  les 
parois  qui  s’enfoncent,  tandis  que  les  parois  du  rectum  sont 
tapisses  d’un  epithelium  cylindrique,  dont  les  cellules  s’aplatis- 
sent  graduellement  vers  les  angles.” 

The  three  pairs  of  Malpighian  vessels  appear  at  a  very  early 
period,  while  the  proctodaeal  invagination  is  still  very  shallow. 
They  are  from  the  first  hollow  diverticula,  and  have  their  blind 
ends  pushed  back  by  the  forward  growth  of  the  proctodaeum. 
Thus  it  happens  that  a  transverse  section  through  the  tail-end 
of  an  embryo  in  the  stage  of  Fig.  74  passes  through  both  the 
proctodaeum  and  the  Malpighian  vessels  (Fig.  79,  ;r,  mpg, 

When  the  blind  ends  of  the  six  tubes  have  struck  the  body  wall, 
their  continued  growth  forces  them  to  turn  and  grow  forward. 
After  the  formation  of  the  dorsal  body  wall,  they  may  be  found 
lying  as  thin  undulating  tubes,  surrounding  the  mesenteron  at 
approximately  equal  distances  from  one  another  (Fig.  86  mp^. 

The  very  early  appearance  of  the  Malpighian  vessels  and 
their  paired  arrangement  in  Doryphora^  Fig.  72,  would  seem  to 


370 


WHEELER. 


[VOL.  III. 


indicate  that  at  one  time  they  opened  on  the  surface  of  the 
body,  and  that  their  orifices  were  subsequently  carried  in  by  a 
deepening  of  the  proctodaeal  invagination.  Possibly  these  tubes 
in  insects  are  homologous  with  the  anal  tubes  of  the  Echiuriis 
larva,  which  are  modified  segmental  organs.  Gegenbauer  (14) 
has  intimated  the  possible  derivation  of  the  Malpighian  vessels 
from  paired  tubes  opening  on  the  outer  surface  of  the  body. 
He  says:  “As  they  [the  Malpighian  vessels]  are  formed  at 
the  same  time  as  that  portion  of  the  hind-gut,  which  in  tfie 
embryo  is  developed  from  the  ectoderm,  it  is  not  improbable 
that  they  primitively  opened  on  the  surface  of  the  body,  or  were 
derived  from  organs  which  did  so.  In  all  divisions  there  are 
two  chief  canals,  as  is  often  seen  at  the  point  where  a  large 
number  of  canals  open  and  unite.  This  number  may  therefore 
be  regarded  as  a  primitive  character.” 

Milwaukee,  Sept.  12,  1888. 


LIST  OF  WORKS  REFERRED  TO. 

\/h.  Ayers,  H.  On  the  development  of  OEcanthus  niveus  and  its  Parasite 
Teleas.  Memoir.  Boston  Soc.  Nat.  Hist.  Vol.  III.,  No.  8.  1884. 

2.  Balfour,  F.  Treatise  on  Comparative  Embryology.  Vol.  II.  1881. 

3.  Blochmann,  F.  Ueber  eine  Metamorphose  der  Kerne  in  den  Ovarial- 

eiern  und  iiber  den  Beginn  der  Blastodermbildung  bei  den  Ameisen. 
Verhand  d.  naturh.  medicin.  Vereins  zu  Heidelberg.  Bd.  III.  Heft 
3.  1884. 

4.  Blochmann,  F.  Ueber  die  Reifung  der  Eier  bei  Ameisen  und  Wespen. 

Festschrift  zur  Feier  des  100  jMir.  Bestehens  der  Ruperto-Carola. 
Heidelberg.  1886. 

5.  Blochmann,  F.  Ueber  die  Richtungskdrperchen  bei  Insekteneiern. 

Morph.  Jahrb.  Bd.  XII.  Heft  4.  1887. 

6.  Brandt,  A.  Ueber  die  Eirdhren  der  Blatta  (Periplaneta)  orientalis. 

Mem.  Acad.  St.  Petersbourg.  Ser.  7.  Tom.  XXL  1874. 

7.  Bruce,  A.  T.  Observations  on  the  Embryology  of  Insects  and  Arachnids. 

Baltimore,  1887. 

8.  Biitschli,  O.  Bemerkungen  iiber  die  Entwicklungsgeschichte  von  Musca. 

Morph.  Jahrb.  Bd.  XIV.  Heft  i.  1888. 

9.  Carnoy,  J.  B.  La  Cytodierese  chez  les  Arthopodes.  “  La  Cellule.”  Tom. 

1.  Fasc.  II.  1885. 

vXio.  Cholodkovsky,  N.  Ueber  die  Bildung  des  Entoderms  bei  Blatta  ger- 
manica.  Zoolog.  Anzeig.  No.  275.  1888. 

Emery,  C.  (Critical  note  on  Korotneff  and  Grass!)  Biolog.  Centralbl. 
Bd.  V.  1887. 


II. 


No.  2.1 

t  -* 


BLATTA  AND  DORYPHORA. 


371 


12.  Flemming,  W.  Zellsubstanz,  Kern  u.  Zelltheilung.  Leipzig,  1882. 

13.  Flemming,  W.  Neue  Beitriige  zur  Kenntniss  der  Zelle.  Arch.  f.  mikr. 

Anat.  Bd.  29.  Heft  3.  1887. 

14.  Gegenbauer,  C.  Elements  of  Comparative  Anatomy.  Trans,  by  F. 

Jeffrey  Bell.  London,  1878. 

/I5.  Graber,  V.  Die  Insekten.  II.  Theil.  Miinchen,  1877. 

16.  Graber,  V.  Ueber  die  primare  Segmentirung  des  Keimstreifs  der  In¬ 
sekten.  Morph.  Jahrb.  Bd.  XIV.  Heft  2.  1888. 

^^17.  Hallez,  P.  Loi  de  I’orientation  de  I’embryon  chez  les  Insects.  Compt. 
Rend.  Tome  103.  1886. 

yi8.  Hatschek,  B.  Beitrage  zur  Entwicklungsgeschichte  der  Lepidopteren. 
Naumburg  a/S.,  1877. 

V^iQ.  Heider,  K.  Ueber  die  Anlage  der  Keimblatter  von  Hydrophilus  piceus. 

Abh.  der  K.  Preuss.  Akad.  d.  Wiss.  z.  Berlin  v.  Jahre  1885. 
y/'  20.  Henking,  H.  Untersuchungen  fiber  die  Entwicklung  der  Phalangiden. 
Theil  1.  Zeitschr.  f.  wiss.  Zool.  Bd.  45.  1886. 

21.  Hertwig,  O.  Welchen  Einfluss  iibt  die  Schwerkraft  auf  die  Theilung 
derZellen?  Jena,  1884. 

,  22.  Huxley,  T.  A.  Manual  of  the  Anatomy  of  Invertebrated  Animals.  1878. 

23.  Kadyi,  H.  Beitrag  zur  Kenntniss  der  Vorgange  beim  Eierlegen  der 

Blatta  orientalis.  Zoolog.  Anzeig.  No.  44.  1879. 

24.  Kennel,  J.  Entwicklungsgeschichte  v.  Peripatus  Edwardsii,  Blanch. 

and  P.  torquatus,  n.  sp.  Arbeit,  aus  d.  zoolog.  zootom.  Instit.  in  Wiirz- 
burg.  Bd.  VII.  Heft  8,  and  Bd.  VIII.  Heft  i.  1886. 

25.  Korotneff,  A.  Die  Embry ologie  der  Gryllotalpa.  Zeitsch.  f.  wiss.  Zool. 

Bd.  41.  Heft  4.  1885. 

■y/26.  Kowalevski,  A.  Embryologische  Studien  an  Wiirmern  u.  Arthropoden. 

Mem.  de  PAcad.  imp.  d.  Sciences  de  St.  Petersbourg.  Ser.  VII.  Tom. 
XVI.  No.  12.  1871. 

27.  Kowalevski,  A.  Zur  Embryonalen  Entwicklung  der  Musciden.  Biolog. 
Centralbl.  Bd.  VI.  No.  2.  1886. 

,^28.  Leichmann,  G.  Ueber  Bildung  von  Richtungskorpern  bei  Isopoden. 
Zoolog.  Anzeig.  No.  262.  1887. 

v^^9.  Leydig,  F.  Beitrage  zur  Kenntniss  des  thierischen  Eies  im  unbefruchte- 
ten  Zustande.  Zoolog.  Jahrbiicher.  Bd.  III.  Heft  2.  Jena,  1888. 

30.  Metschnikow,  E.  Embryologische  Studien  an  Insecten.  Leipzig, 

1866. 

31.  Metschnikow,  E.  Embryologie  der  doppeltfiissigen  Myriapoden  (Chil- 

ognatha).  Zeit.  f.  wiss.  Zool.  Bd.  XXIV.  1874. 

32.  Miall,  L.  C.,  and  Denny,  A.  The  Structure  and  Life  History  of  the 

Cockroach  (Periplaneta  orientalis).  London,  1886. 

■^33.  Nusbaum,  J.  The  Embryonic  Development  of  the  Cockroach  (Blatta 
germanica)  (in  the  work  of  Miall  &  Denny). 

34.  Nusbaum,  J.  L’embryologie  de  Mysis  chameleo,  Thomp.  Arch.  Zool. 
Exper.  TomeV.  1887. 

^  35.  Packard,  A.  S.  The  Embryology  of  Chrysopa  and  its  Bearings  on  the 
Classification  of  the  Neuroptera.  Am.  Naturalist.  Vol.  V.,  p.  564. 
1871. 


372 


WHEELER. 


36.  Palm^n,  J.  A.  Morphologie  des  Tracheensystems.  1877. 

37.  Palm^n,  J.  A.  Ueber  paarige  Ausfuhrungsgange  der  Geschlechtsorgane 

bei  Insekten.  Helsingfors,  1884. 

38.  Patten,  W.  The  Development  of  Phryganids,  with  a  preliminary  note 

on  the  Development  of  Blatta  germanica.  Quart.  Journ.  Mic.  Science. 
Vol.  XXIV.  1884. 

^39.  Patten,  W.  Studies  on  the  Eyes  of  Arthropods.  II.,  Eyes  of  Acilius. 
Journal  of  Morph.  Vol.  II.  No.  i.  1888. 

40.  Pereyaslawzewa,  Sophie.  Le  developpement  de  Gammarus  poecilurus, 

Rthk.  Bulletin  d.  la  Soc.  Imp.  d.  Naturalistes  d.  Moscou.  Annde 
1888.  No.  2. 

41.  Rabl,  C.  Ueber  Zelltheilung.  Morph.  Jahrb.  Bd.  X.  Heft  2.  1884. 

42.  Ratlike,  H.  Zur  Entwicklungsgeschichte  der  Blatta  germanica.  Meckel’s 

Archiv  f.  Anat.  u.  Physiol.  Bd.  VI.  1882. 

43.  Sedgwick,  A.  On  the  Origin  of  Metameric  Segmentation,  and  some 

other  Morphological  Questions.  Stud,  from  the  Morph.  Lab.  of  Cam¬ 
bridge.  Vol.  II.  Pt.  I.  1884. 

Strasburger,  E.  Zellbildung  und  Zelltheilung.  3  Auflage.  Jena,  1880. 
45.  Stuhlmann,  F.  Die  Reifung  des  Arthropodeneies  nach  Beobachtungen 
an  Insekten,  Spinnen,  Myriapoden  u.  Peripatus.  Bericht  d.  naturf. 
Gesellsch.  z.  Freiburg  i/Br.  Bd.  i.  Hefte  5,  12.  1886. 

\/ 46.  Taschenberg,  E,  L.  Blatta  germanica.  In  Brehm’s  Thierleben.  Vol. 
IX.,  p.  536.  Leipzig,  1884. 

47.  Weismann,  A.  Beitrage  zur  Kenntniss  der  ersten  Entwicklungsvorgange 
im  Insektenei.  Bonn,  1882. 

V  48.  Weismann,  A.  Ueber  die  Zahl  der  Richtungskorper  und  ihre  Bedeutung 
fiir  die  Vererbung.  Jena,  1887. 

>^  49-  Weismann,  A.,  and  Ischikawa,  C.  Ueber  die  Bildung  der  Richtungs- 
kbrper  bei  thierischen  Eiern.  Berichte  d.  naturf.  Gesell.  z.  Freiburg 
i/Br.  Bd.  III.  Heft  i.  1887. 

^  50.  Wheeler, W.  M.  On  the  Appendages  of  the  First  Abdominal  Segment 
of  the  Embryo  Cockroach  (Blatta  germanica).  Proceed.  Wis.  Acad. 
Science,  Arts  and  Letters.  Vol.  VIII.  1888. 

51.  Will,  L.  Bildungsgeschichte  und  morphologischer  Werth  des  Eies  v. 

Nepa  cinerea,  L.  u.  Notonecta  glauca,  L.  Zeitschr.  f.  wiss.  Zool.  Bd. 
41.  Heft  3.  1885. 

52.  Will,  L.  Entwicklungsgeschichte  der  viviparen  Aphiden.  Zoolog.  Jahrb. 

Bd.  III.  Heft  2.  1888. 


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374 


WHEELER. 


DESCRIPTION  OF  PLATE  XV. 

Blatta  ger7nanica. 

Fig.  I.  Piece  of  chorion  from  lateral  face  of  oothecai  egg. 

Fig.  2.  Piece  of  chorion  from  micropylar  area  with  micropyles.  a,  aperture  of 
micropyle;  b,  tubule. 

Fig.  3.  Transverse  section  through  the  crista  of  a  capsule.  0,  oothecai  wall  split¬ 
ting  into  two  laminae,  6^  and  cr,  crystals  of  calcium  oxalate;  ep,  remains  of  fol¬ 
licular  epithelium. 

Fig.  4.  Longitudinal  section  of  epithelial  cap  on  the  germarium  pole  of  the  egg. 
ep,  follicular  epithelium;  ch,  chorion  secreted  by  the  same;  a,  the  point  at  which  the 
chorion  widens,  showing  its  lengthened  trabeculae;  b,  pyriform  dilatation  of  the  chorion; 
Pr,  protoplasm  of  the  Keinhaut,  containing  numerous  bacteria-like  corpuscles. 

Fig.  5.  A  number  of  cells  from  the  follicular  epithelium.  «,  resting  nucleus;  b 
and  c,  nuclei  in  akinetic  division. 

Fig.  6.  Longitudinal  section  of  immature  ovum  i  mm.  long,  being  the  circum- 
nuclear  portion  in  the  centre  of  the  dorsal  concavity  of  the  egg;  nucleus  amoeboid 
just  after  reaching  the  surface,  n,  chromatin  particles;  x,  layer  of  small  yolk  bodies 
just  beneath  the  epithelium. 

Fig.  7.  Same  section  of  an  egg  2  mm.  long.  Nucleus  emarginate  on  outer  surface, 
giving  off  the  maturation  sphere  b-,  y,  small  yolk  bodies  under  the  epithelium.. 

Fig.  8.  Section  at  right  angles  to  that  in  Fig.  7,  of  a  nucleus  in  the  same  stage. 
b,  cavity  into  which  the  maturation  spheres  fitted;  pn,  paranucleolus. 

Fig.  9.  Nucleus  in  the  same  stage;  pn,  paranucleolus.  Egg  2  mm.  long;  section 
in  the  same  plane  as  Fig.  7. 

Fig.  10.  Longitudinal  section  of  median  dorsal  surface  of  an  egg  2.5  mm.  long. 
Particles  of  chromatin  aggregating  into  a  somewhat  stellate  central  mass. 

Fig.  II.  Longitudinal  section  corresponding  to  Fig.  10  of  an  egg  somewhat  older, 
2.8  mm.  long.  A  delicate  wall  encloses  the  particles  of  chromatin. 

Fig.  12.  Pdrst  polar  spindle  in  the  metakinetic  stage.  Egg  mature. 

Fig.  13.  First  polar  spindle  in  anaphasis.  Egg  mature. 

Fig.  14.  Median  longitudinal  section  through  the  dorsal  surface  of  an  oothecai 
egg  4  to  6  hours  old.  pgl ,  first  polar  globule;  x,  second  polar  spindle  in  metakinesis. 

Fig.  15.  Longitudinal  section  through  botl^  polar  globules,  3  to  4  hours  older 
than  egg  represented  in  Fig.  14. 

Fig.  16.  Surface  view  of  the  median  dorsal  portion  of  an  oothecai  egg  about  10 
hours  old.  pgl  pgl,  polar  globules;  9  P^h  female  pronucleus  (out  of  focus). 

Fig.  17.  Median  longitudinal  section  of  dorsal  portion  of  an  oothecai  egg.  a, 
dorsal  contour;  J  p7t,  female  pronucleus  just  after  leaving  the  polar  globules. 

Fig.  18.  Longitudinal  section  of  circumnuclear  region,  from  near  the  middle  of 
the  homogeneous  yolk,  with  female  pronucleus.  The  arrow  in  this  and  the  following 
figures  points  in  the  direction  of  the  cephalo-caudal  axis  of  the  egg. 

Fig.  19.  Longitudinal  section  of  circumnuclear  portion  of  egg,  one-third  the  length 
of  the  egg  from  the  cephalic  pole,  a,  dorsal  contour;  9  female  ^ronucleus; 
^  pn,  male  pronucleus. 

Fig.  20.  Longitudinal  section  of  circumnuclear  area  from  middle  of  homogeneous 
yolk.  $  pn  and  9  male  and  female  pronuclei  conjugating. 

Fig.  21.  Longitudinal  section  of  circumnuclear  area  from  the  middle  of  the  front 
portion  of  the  homogeneous  yolk.  Cleavage  nucleus. 

Fig.  22.  Cleavage  nucleus  preparing  to  divide,  cp,  cytoplasm. 


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WHEELER. 


DESCRIPTION  OF  PLATE  XVI. 

Blatta  germanica. 

Fig.  23.  Cleavage  nucleus  dividing,  a,  points  to  the  ventral  contour.  The  nucleus 
in  this  egg  had  moved  out  into  the  granular  yolk  some  distance. 

Fig.  24.  Two  nuclei;  a,  seen  in  lateral,  and  b,  in  polar  view,  from  an  egg  contain¬ 
ing  4  nuclei  in  the  metakinetic  stage  of  division. 

Fig.  25.  Ventral  third  of  a  transverse  section  of  an  odthecal  egg;  a  and  b,  cells 
which  have  just  reached  the  surface.  The  amoeboid  cell  in  the  interior  is  on  its  way 
to  the  surface. 

Fig.  26.  Corresponding  section  of  the  somewhat  older  egg  (Fig.  36).  a,  a 
group  of  cells  formed  by  tangential  akinesis  from  one  of  the  nuclei,  like  a,  Fig.  25. 

Fig.  27.  Ventral  third  of  a  transverse  section  of  an  egg  in  the  blastoderm  stage; 
6  days  old. 

Fig.  28.  Ventral  third  of  a  transverse  section.  Blastoderm  contracting.  At  3r', 
yolk  nucleus  being  given  off;  at  x,  one  which  has  sunk  deep  into  the  yolk. 

Fig.  29.  Transverse  section  through  e,  of  an  embryo  like  the  one  represented  in 
Fig.  43,  V,  yolk-cell;  msd,  mesoderm;  ecd.,  ectoderm. 

Fig.  30.  Transverse  section  through  d  of  the  embryo  43.  am,  amnion;  sr,  serosa; 
ecd,  ectoderm;  msd,  mesoderm;  v,  yolk-cell. 

Fig.  31.  Transverse  section  through  the  point  pd  of  Fig.  41.  ecd,  ectoderm;  v, 
yolk-cell;  i7isd,  mesoderm. 

Fig.  32.  Transverse  section  through  a  point  a  little  in  front  of  as  of  Fig.  43.  am, 
amnion;  sr,  serosa;  ecd,  ectoderm;  7nsd,  mesoderm;  v,  yolk-cell. 

Fig.  33.  Surface  view  of  blastoderm,  showing  the  binucleolate  nuclei. 

Fig.  34,  Enlarged  view  of  syncytium  x,  in  Fig.  36;  showing  the  unequal  size  of 
the  nuclei. 

Fig.  35.  a  and  c,  surface  nuclei  in  akinetic  division  from  an  egg  in  the  same  stage 
as  Fig.  36;  b,  serosa  nucleus  in  akinesis  from  a  much  older  egg. 

Fig.  36.  Surface  view  of  an  egg  shortly  after  the  nuclei  have  begun  to  appear 
on  its  surface,  c,  cephalic  end;  s,  sinus;  v,  ventral  surface;  dd,  dorsal  surface;  x,  a 
group  of  nuclei  like  that  seen  in  section  at  a,  in  Fig.  26. 

Fig.  37.  Surface  view  of  an  egg  in  the  blastema  stage;  4  days  old. 

Fig.  38.  Long.  sect,  along  the  carina  cn,  of  Fig.  41;  through  the  depression  bp. 
sr,  serosa;  bp,  depression;  ecd,  ectoderm;  msd,  mesoderm;  v,  yolk-cells. 


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WHEELER. 


DESCRIPTION  OF  PLATE  XVII. 

Blatta  gernianica. 

Fig.  39.  Diagram  of  a  median  longitudinal  section  of  an  egg  to  show  paths  of 
nuclei.  Explanation  in  the  text. 

Fig.  40.  Diagram  of  a  transverse  equatorial  section  of  an  egg  to  show  paths  of 
nuclei.  Explanation  in  the  text. 

Fig.  41.  Surface  view  of  the  ventral  face  of  an  egg  *j\  days  old.  cn,  carina;  pd^ 
beginnings  of  procephalic  lobes;  hp,  depression  (blastopore?)  in  the  middle  of  the 
rounded  area  of  proliferation. 

Fig.  42.  Surface  view  of  the  ventral  face  of  an  egg  8  days  old.  as,  amnion  and 
serosa  beginning;  pd,  beginning  of  procephalic  lobes. 

Fig.  43.  Surface  view  of  an  embryo  in  “  slipper  ”  stage,  as,  amnion  and  serosa. 

Fig.  44.  Surface  view  of  an  embryo  in  the  “  hammer”  stage,  9|  days  old.  0,  open¬ 
ing  in  amnion  and  serosa  over  the  oral  region;  at,  antennary  lobe;  p,  beginning  of 
the  second  thoracic  appendage. 

Fig.  45.  Embryo  about  ii  days  old,  isolated  from  the  yolk,  lb,  labrum;  st,  stomo- 
dseum;  brain;  antennary  lobe;  mandible;  ist  and  2d  maxillae; 

p^  to  p^,  thoracic  appendages;  as,  amnion  and  serosa;  cpl,  caudal  plate. 

Fig.  46.  Embryo  just  after  rupture  of  amnion  and  serosa;  about  14  days  old.  b, 
cephalic  end  of  yolk;  oc,  eye;  ad,  fat  body,  ast,  anal  stylets;  appendages  of  ist 
abdominal  segment;  remaining  references  same  as  in  Fig.  45. 

Fig.  47,  Front  view  of  same  embryo.  References  the  same  as  in  Fig.  45. 

Fig.  48.  Embryo  almost  ready  to  hatch,  oc,  eye;  cgl,  brain;  gl'^,  infraoesophageal 
ganglion;  gl^  to  the  three  thoracic  ganglia;  gl^,  last  of  the  abdominal  ganglia; 
oe,  oesophagus;  c,  crop;  gz,  gizzard;  ht,  heart;  st,  stomach;  ret,  rectum;  mpg,  Mal¬ 
pighian  vessels;  ast,  anal  stylets;  y,  yolk;  lb,  labrum;  nid,  mandibles;  inx^  and 
vix^,  1st  and  2d  maxillae;  p^  to  p^,  thoracic  appendages;  at,  antennae. 

Fig.  49.  Blatta  germanica  about  a  day  after  hatching. 

Fig.  50.  Sagittal  longitudinal  section  through  the  middle  of  the  dorsal  organ  from 
an  embryo  16  days  old.  a,  anterior;  b,  posterior  end  of  dorsal  organ;  0,  depression; 
V,  a  yolk  nucleus. 

Fig.  51.  Frontal  section  through  the  anterior  end  of  dorsal  organ  in  an  embryo  18 
days  old.  cc,  heart;  nn,  nuclei  of  serosa  cells;  0,  cavity  formed  from  the  depression 
0,  of  Fig.  50;  V,  yolk  nucleus. 

Fig.  52.  Cross-section  through  the  dorsum  of  an  embryo  to  show  formation  of  the 
heart.  /^,  cavity  of  heart;  cardioblasts;  ectoderm;  somatic  mesoderm; 

slm,  splanchnic  mesoderm;  ent,  entoderm;  bl,  blood  corpuscle  becoming  loosened 
from  the  wall  of  the  heart;  y,  yolk. 

Fig.  53.  Cross-section  through  the  thorax  of  an  embryo  12  days  old.  nr,  neural 
thickening;  am,  amnion;  sr,  serosa;  v,  yolk  nucleus;  nisd,  mesoderm;  cl,  coelomic 
cavity;  p^^,  second  pair  of  legs. 

Fig.  54.  Cross-section  through  2d  maxillcC  of  an  embryo  16  days  old.  mx^,  second 
maxilla;  g,  duct  of  salivary  gland;  y,  yolk;  ent,  entoderm;  cb,  cardioblast  cells; 
remainder  of  references  as  in  Fig.  53. 

Fig.  55.  Cross-section  through  the  basal  abdominal  somite  of  an  embryo  26  days 
old.  gl,  ganglion;  ad,  corpus  adiposum;  v,  yolk  nucleus;  tr,  tracheal  opening; 
yolk;  cc,  heart. 


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B.MeiselJilh.Boston . 


38o 


WHEELER. 


DESCRIPTION  OF  PLATE  XVIII. 

Doryphora  decemlineata. 

Fig.  56.  Portion  of  a  longitudinal  section  through  a  young  egg  shortly  after  the 
germinal  vesicle  has  reached  the  periphery,  pi,  karyoplasm;  tr,  protoplasmic  trabe¬ 
culae  connecting  the  karyoplasm  with  the  intervitelline  protoplasm  (cytoplasm  of  the 
large  cell  which  the  egg  represents);  y,  yolk  bodies;  ep,  follicular  epithelium;  nl, 
germinal  spot  (nucleolus) ;  mb,  maturation  balls  (Reifungsballen  of  Stuhlmann)  Zeiss 
oc.  III. 

Fig.  57.  Section  corresponding  to  that  of  Fig.  56  from  an  egg  almost  mature. 
n,  large  mass  of  chromatin  destined  to  enter  into  the  first  polar  spindle;  bl,  layer  of 
surface  protoplasm;  y,  yolk  bodies;  ep,  follicular  epithelium.  Zeiss  oc.  III. 

Fig.  58.  Longitudinal  section  of  a  mature  egg.  resting  nucleus  originating  from 
the  mass  of  nuclein  n  of  Fig.  57;  bl,  differentiated  peripheral  layer  of  protoplasm; 
y,  yolk  bodies;  v,  vitelline  membrane;  ch,  chorion.  Zeiss 

Fig.  59.  Portion  of  median  transverse  section  of  a  mature  egg.  n,  first  polar  spindle 
in  metakinetic  stage;  remaining  references  the  same  as  in  Fig.  58.  Zeiss  oc.  III. 

Fig.  60.  Same  section  as  in  Fig.  59  of  an  egg  about  to  be  deposited,  nucleus 
in  last  stages  of  ahaphasis;  the  peripheral  mass  the  first  polar  globule.  Remaining 
references  the  same  as  in  Fig.  58.  Zeiss  jLj  oc.  III. 

Fig.  61.  Half  of  the  median  transverse  section  of  an  egg  containing  several  nuclei 
n^,  n^,  none  of  which  have  as  yet  entered  the  surface  layer  of  protoplasm  bl.  y,  yolk. 

Fig.  62.  Half  of  the  median  transverse  section  of  an  egg  shortly  before  the  blasto¬ 
derm  stage.  V,  yolk-cells;  a,  blastema  cell  resting;  b,  blastema  cells  in  metakinesis; 
c,  blastema  cells  in  anaphasis;  y,  yolk. 

Fig.  63.  Half  of  the  median  transverse  section  of  an  egg  in  the  blastoderm  stage. 
V,  yolk-cells;  bid,  blastoderm. 

Fig.  64.  Half  of  a  median  transverse  section  of  the  egg  of  Fig.  67.  g,  half  of  the 
gastrular  groove  forming  on  the  ventral  face;  r,  ridge  separating  the  groove  from  the 
remainder  of  the  blastoderm;  p,  flat  blastoderm  cells  on  the  dorsal  surface  of  the  egg. 

Fig.  65.  Median  transverse  section  of  an  egg  carrying  the  embryo  represented  in 
Fig.  71.  The  upper  half  of  the  section  passes  through  p^  of  Fig.  71,  the  lower  half  a 
little  in  front  of  x.  sr,  serosa;  am,  amnion;  ecd,  ectoderm;  msd,  mesoderm;  ent, 
cells  still  forming  part  of  the  induplicated  ectoderm  at  pi.  which  give  rise  to  the  ento¬ 
derm;  y,  yolk. 

Fig.  66.  Ventral  view  of  egg  shortly  after  formation  of  blastoderm;  the  two  paren¬ 
thesis-shaped  ridges  inclose  the  portion  which  will  sink  in  to  form  the  gastrula. 

Fig.  67.  Ventral  view  of  egg  with  gastrula  more  advanced,  a,  oral  spade-shaped 
broadening  of  the  gastrula;  b,  the  point  at  which  the  groove  turns  abruptly  inwards; 
sr,  serosa. 

Fig.  68.  Lateral  view  of  same  egg.  pci,  procephalic  lobes;  remaining  references 
the  same  as  in  Fig.  67. 

Fig.  69.  Caudal  end  of  egg  represented  in  Fig.  70.  g,  gastrula;  am,  caudal  fold 
of  the  amnion;  sr,  serosa. 

Fig.  70.  Ventral  view  of  an  egg  with  gastrula  fully  formed,  a,  oral  widening  of 
the  gastrula;  am,  cephalic  fold  of  amnion;  br,  brain  thickening. 

Fig.  71.  Slipper-shaped  embryo  removed  from  egg  and  unrolled,  p^  to  p^,  indica¬ 
tions  of  the  3  pairs  of  thoracic  appendages;  as,  amnion  and  serosa;  am,  cephalic 
fold  of  amnion  and  serosa;  a,  oral  and  x,  anal  end  of  gastrula;  b,  maxillary  region. 


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WHEELER. 


DESCRIPTION  OF  PLATE  XIX. 

Doryphora  decemlineata. 

Fig.  72.  Embryo  shortly  after  the  appearance  of  the  appendages,  unrolled  and  iso¬ 
lated.  c,  stomodaeum;  a,  proctodseum;  lb,  labrum;  to  three  brain  segments; 
og^  to  og^,  three  segments  of  the  optic  ganglion;  op^  to  op^,  three  segments  of  the 
optic  plate;  to  five  pairs  of  invaginations  which  form  the  tentorium;  f  to 
tracheal  invaginations;  the  two  last  pairs  and  either  disappear  or  form  the 
openings  of  the  sexual  ducts;  at,  antennce;  nid,  mandibles;  inx^  and  mx^,  first  and 
second  maxillae;  to  p^,  three  pairs  of  thoracic  appendages;  c,  commissure  con¬ 
necting  the  two  ganglionic  thickenings  g'^  of  the  intercalary  segment;  gl,  ganglionic 
thickening;  mst,  Mittelstrang  thickening;  mpg^  to  nipg'^,  three  Malpighian  vessels. 

Fig.  73.  Embryo  just  after  the  closing  of  the  amnion  and  serosa  (jr)  unrolled  and 
isolated,  a,  oral,  x,  anal  end  of  the  gastrula;  pci,  procephalic  lobe;  nix^^,  second 
maxilla;  p^  to/^,  three  thoracic  appendages;  cp,  caudal  plate. 

Fig.  74.  Ventral  view  of  embryo,  the  lateral  walls  of  which  have  embraced  half 
the  yolk;  references  same  as  in  Fig.  72. 

Fig.  75.  Lateral  view  of  embryo  ready  to  hatch,  at,  antenna;  mesothoracic 
spiracle;  to  abdominal  spiracles;  hsp,  first  hatching-spine. 

Fig.  76.  Sagittal  section  to  one  side  of  the  median  line  through  the  tail  of  embryo. 
Fig.  72.  a?n,  amnion;  sr,  serosa;  ?7ipg'^  and  mpg^,  first  and  third  Malpighian 
vessels;  ecd,  ectoderm;  7Jisd,  mesoderm;  oit,  forward  growing  band  of  entoderm; 
e7if^,  mass  of  entoderm  left  under  the  end  of  the  proctodseum. 

Fig.  77.  Transverse  section  through  the  second  pair  of  Malpighian  vessels  (77ipg‘^) 
of  the  same  embryo,  x,  proctodaeum;  other  references  same  as  in  Fig.  76. 

Fig.  78.  Median  cross-section  through  an  egg  containing  an  embryo  a  little  older 
than  that  represented  in  Fig.  72.  The  section  cuts  the  embryo  at  two  places.  That 
with  the  entoderm  belongs  to  one  of  the  last  abdominal  segments,  that  without 

entoderm  to  one  of  the  basal  abdominal  segments,  wr,  neural  thickenings;  ecd  and 
eiit,  ectoderm  and  entoderm;  sh7i,  S7n,  splanchnic  and  somatic  mesoderm,  inclosing 
the  coelomic  cavity;  cb,  cb,  cardioblast  cells. 

Fig.  79.  Cross-section  through  the  middle  of  the  proctodaeum  of  an  embryo  some¬ 
what  younger  than  that  represented  in  Fig.  74.  x,  proctodaeal  cavity;  cl,  coelomic 
cavity;  gl,  ganglion;  771st,  Mittelstrang;  77tp^  to  7npg^,  Malpighian  vessels  sur¬ 
rounded  by  mesodermic  elements. 

Fig.  80.  Cross-sections  through  the  sexual  orifices  of  an  embryo  in  the  stage  of 
Fig.  74.  ^(9,  ^(3,  openings  of  efferent  ducts;  remaining  references  as  in  preceding 
figures. 

Fig.  81.  Surface  view  of  two  eyes  (corresponding  to  eyes  V.  and  VI.  of  Acilius  as 
described  by  Patten).  71,  large  nucleus  in  the  centre  of  each  eye. 

Fig.  82.  Median  sagittal  section  through  an  embryo  in  about  the  stage  represented 
in  Fig.  73.  aTTi  and  sr,  amnion  and  serosa;  0,  point  at  which  the  anterior  end  of 
the  embryo  passes  into  the  amnion;  p,  point  at  which  the  posterior  end  passes  into 
the  amnion;  st,  stomodaeal  depression;  x,  point  near  which  the  proctodseum  will 
appear;  pl'^,  pl'^,  anterior  and  posterior  thickenings  of  the  inner  layer;  from  which 
the  entoderm,  eTit^,  eTit^,  originate.  Tnsd,  mesoderm;  77ix^  and  mx‘^,  first  and  second 
maxillae;  p^  to  p^,  legs;  c,  cells  which  entered  the  amniotic  cavity.  Under  the  two 
thickenings  pL,  pl'^,  are  seen  the  peculiar  degenerating  nuclei  passing  into  the  yolk, 
which  has  become  segmented. 


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Joiini.  Mor/i/i  Vo i. III. 


PL  XIX 


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B.MeiselJith.BostQn. 


WHEELER, 


0 


84 


DESCRIPTION  OF  PLATE  XX. 

Doryphoj'a  decemlineata. 

Fig.  83.  Portion  of  cross-section  through  a  basal  abdominal  segment  of  an  em¬ 
bryo  somewhat  younger  than  that  represented  in  Fig.  74.  am,  amnion;  ent,  entoderm; 
ect,  ectoderm;  tr,  tracheal  invagination;  gl,  ganglion;  nist,  Mittelstrang;  sni,  somatic 
mesoderm;  slm,  splanchnic  mesoderm;  cb,  cardioblasts. 

Fig.  84.  Transverse  section  through  middle  of  abdomen  of  embryo,  Fig.  74.  gn^ 
sexual  organ;  mpg,  Malpighian  vessel;  remaining  references  same  as  in  Fig.* 83. 

Fig.  85.  Cross-section  through  metathoracic  segment  of  an  embryo  considerably 
older  than  Fig.  74.  In,  lateral  nerve  trunk;  ap,  appendage;  sr,  serosa;  ad,  fat  cells; 
a,  amnion  cells  entering  the  yolk  at  b  ;  remaining  letters  the  same  as  in  Fig.  83. 

Fig.  86.  Cross-section  through  mesothorax  of  an  embryo  after  the  formation  of 
the  heart,  ht.  hsp,  thickening  of  ectoderm  which  secretes  the  metathoracic  hatching- 
spine;  cP,  first  cuticle  shed  by  the  embryo;  cm,  longitudinal  commissures  between 
two  thoracic  ganglia;  f,  furca;  r,  point  of  attachment  of  muscles  to  the  furca;  msl, 
muscular  tissue;  remaining  letters  as  in  preceding  figures. 

Fig.  87.  Cross-section  through  caudal  plate  of  embryo  before  the  closure  of  the 
anal  end  of  the  gastrula,  x.  c,  cell  about  to  wander  through  the  blastopore  into  the 
amniotic  cavity;  msd,  mass  of  cells  which  will  form  the  mesoderm  as  soon  as  the  blas¬ 
topore  closes;  ent,  cells  imperfectly  differentiated  from  those  of  the  mass,  msd,  which 
are  to  give  rise  to  the  entoderm;  v,  yolk  nucleus;  am,  amnion;  sr,  serosa. 

Fig.  88.  Cross-section,  more  highly  magnified  than  the  preceding,  through  the 
caudal  plate  of  an  embryo  somewhat  younger  than  that  of  Fig.  73.  The  blastopore 
has  closed,  and  the  depression  x  marks  the  beginning  of  the  proctod^al  invagina¬ 
tion;  /  is  the  line,  unusually  distinct  in  the  section  figured,  separating  the  mass  of 
mesoderm  cells  (msd)  above  it  from  the  smaller  mass  of  entoderm  cells  (ent)  below 
it;  t,  0,  V,  degenerating  nuclei  which  pass  from  the  entoderm  into  the  yolk;  y,  yolk 
nucleus. 

•  Fig.  89.  Frontal  section  through  the  cardioblasts  (cb)  of  an  embryo  in  the  same 

stage  as  Fig.  84;  cb,  row  of  cardioblasts;  ecd,  msd,  ent,  germ  layers;  ad,  fat  cells. 

Fig.  90.  Portion  of  a  cross-section  through  the  lower  part  of  the  dorsal  yolk  of 
an  embryo  in  the  stage  of  Fig.  74.  ch,  chorion;  sr,  serosa;  am,  amnion;  do,  amniotic 
dorsal  organ;  m,  amniotic  nuclei  degenerating;  v,  yolk  nucleus. 

Fig.  91.  Frontal  section  through  the  meso-  and  meta-thoracic  ganglia  of  the  em¬ 
bryo,  Fig.  74.  cm,  longitudinal  commissures;  ccm,  cross-commissures;  fP,  anterior 
nerve  trunk;  fp,  posterior  nerve  trunk;  onrl,  outer  neurilemma;  inrl,  inner  neu¬ 
rilemma. 


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WM.Wheaiec.  dtl. 


B-Meisellilh.Boston. 


386 


WHEELER. 


DESCRIPTION  OF  PLATE  XXI. 

Doryphora  decenilineata. 

Fig.  92.  Sagittal  section  through  stomodaeum  {st)  of  an  embryo  in  the  same  stage 
as  Fig.  72.  ecd,  ent,  msd,  germ  layers;  lb,  labrum. 

Fig.  93.  Cross-section  of  median  dorsal  portion  of  an  embryo,  just  after  the  union 
of  the  ectoderm  {ecd')  from  either  side  of  the  body;  cb,  cardioblasts ;  ad,  fat  cells; 
am,  one  of  the  last  amnion  cells  passing  into  the  yolk  through  the  opening  between 
the  two  lateral  halves  of  the  mesenteron.  msl,  muscular  tissue;  ent,  entoderm;  slm, 
splanchnic  mesoderm. 

Figs.  94  to  104.  Sections  i,  3  to  8,  10,  ii,  13,  and  14,  through  the  mesothoracic 
ganglion  of  an  embryo  somewhat  younger  than  that  of  Fig.  74.  cm,  longitudinal 
commissure;  cross-commissures;  /j,  Punktsubstanz;  Mittelstrang;  ecd 
e,  hypodermis;  gbl,  ganglioblasts. 

Fig.  105.  Section  through  fore  part  of  metathoracic  ganglion.  The  Mittelstrang, 
mst,  persists  as  the  furca  (subsequently  chitinous) .  gl,  ganglion;  ps,  Punktsubstanz; 
msl,  muscular  tissue  attached  to  the  furca;  ecd,  hypodermis. 


Joiirn .  Miu/ih  .  Vol.JII. 


PL  XXI 


crrt. 


JOS. 


Ur.V.H7ree/cr.  dti. 


B^MeiseJ.lUh.  Boston . 


A  ('ONTRIl-iUTlON 


'!'(  1 


0 


Insect  Embryology 


■V 


AX  IXAlXiURAL  IMSSERTATIOX 


FOR  THE 


DEGREE  OF  DOCTOR  OF  PHILOSOPHY, 


PRESENTED  TO  THE 


FACULTY  OF  CLARK  UNIVERSITY, 


A/ay  10^  i8g2. 


BY 


WILLIAM  MORTON  WHEELER. 


Reprinted  from  JOURNAL  OF  Morphology,  VoI.  VIII.,  No/  l. 


BOSTON  : 

GINN  <&  CONIF»ANY. 

1893. 


?  'k 


■i  ' 


A  CONTRIBUTION 


Insect  Embryology. 


AN  INAUGURAL  UISSERTATI(3N 

FOR  THE 


DEGREE  OE  DOCTOR  OE  PHILOSOPHY, 

PRESENTED  TO  THE 

FACULTY  OF  CLARK  UNIVERSITY, 

May  lo,  i8g2. 


BY 


WILLIAM  MORTON  WHEELER. 


Reprinted  from  Journal  of  Morphology,  Vol.  VIIL,  No.  i. 


BOSTON : 

OINN  &  CONlF*ANY. 
1893. 


Volume  VIIL 


April,  l8gj. 


Number  /. 


JOURNAL 

OF 

MORPHOLOGY. 


A  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 

WILLIAM  MORTON  WHEELER. 


TABLE  OF  CONTENTS. 

Page 

Introduction  .  2 

I.  The  embryonic  development  of  the  Locustidas .  3 

1.  The  oviposition  of  Xiphidium  ensiferum,  Scud. .  3 

2.  The  formation  of  the  Xiphidium  embryo  and  its  backward  passage 

through  the  yolk  .  3 

(a)  Description  of  surface  changes  beginning  with  the  completed 

blastoderm  .  3 

(b)  The  indusium  {prceoral  organ)  in  section  .  12 

3.  The  development  of  the  embryo  from  the  time  of  its  reaching  the 

dorsal  stirface  of  the  yolk  to  revolution .  18 

4.  Variations  in  the  development  of  the  indusium .  23 

3.  The  revolution  of  the  embryo  .  27 

6.  The  stages  intervening  between  revolution  and  hatching. .  29 

7.  The  develop7nent  of  Orchelimu7n  vulgare .  35 

II.  Gastrulation  in  the  Orthoptera .  36 

III.  The  indusium  and  its  homologues  in  the  Arthropoda .  33 

IV.  General  consideration  of  the  embryonic  envelopes  and  revolution  of 

the  insect  embryo . - .  59 

1.  The  amnion  and  serosa .  59 

2.  The  yolk . 64 

3.  Blastokinesis . 67 

4.  The  elimination  of  the  embryonic  envelopes . .  78 


2 


WHEELER. 


[VoL.  VIIL 


V.  Neurogenesis  in  the  Insecta .  82 

1.  The  nerve-cord .  82 

2.  The  bram  .  99 

3.  General  remarks  on  the  nervous  system .  108 

VI.  The  development  of  the  reproductive  organs  in  the  Insecta .  113 

1.  The  go7iads .  113 

2.  The  male  ducts .  116 

3.  The  female  ducts .  119 

4.  General  considerations .  126 

VII.  The  subcesophageal  body  in  Xiphidium  and  Blatta .  136 

VIII.  Technique .  139 

IX.  Bibliography .  143 

Description  of  the  plates .  150-160 


The  very  primitive  and  synthetic  character  of  the  Orthoptera 
has  long  been  recognized  by  systematists  and  comparative 
anatomists,  but  the  full  importance  of  the  group  from  an 
embryological  standpoint  has  been  but  little  appreciated,  owing 
to  the  meagre  and  fragmentary  nature  of  the  observations 
hitherto  published.  For  this  reason  I  have  made  the  Orthop¬ 
tera  the  starting  point  of  my  studies,  with  a  view  to  determining 
their  relations,  on  the  one  hand  to  the  Apterygota  and  on  the 
other  to  the  higher  Pterygote  orders.  Only  a  portion  of  the 
evidence  bearing  on  these  relationships  is  presented  in  the 
following  paper  ;  a  number  of  observations  on  the  Malpighian 
vessels,  corpus  adiposum,  oenocyte-clusters  and  abdominal  ap¬ 
pendages  will  be  published  as  separate  papers. 

I  have  devoted  more  attention  to  Xiphidittm  than  to  other 
Orthoptera,  partly  because  the  Locustidae  occupy  a  somewhat 
central  position  in  the  order,  and  partly  because  this  curious 
form  exhibits  in  its  embryogeny  better  than  any  other  insect 
hitherto  studied,  the  co-existence  of  certain  very  ancient  with 
very  modern  characters. 

My  German  co-workers  in  the  field  of  insect  development 
will  probably  regard  my  treatment  of  the  literature  as  rather 
perfunctory  ;  but  Prof.  Graber,  Dr.  Heider  and  others  have 
given  from  time  to  time  such  complete  resumes  of  past  and 
current  literature  that  I  feel  justified  in  departing  from  the 
general  custom.  If  I  have  failed  to  give  credit  where  it  is  due, 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  3 

I  beg  that  this  may  be  regarded  as  a  fault  of  omission  and  not 
as  a  fault  of  commission. 

I  would  express  my  sincere  gratitude  to  Prof.  C.  O.  Whit¬ 
man  for  his  kindly  guidance  and  friendly  counsel  throughout 
the  progress  of  my  work  in  his  laboratory  at  Clark  University 
during  the  autumn  and  winter,  and  at  the  Marine  Biological 
Laboratory  during  the  summer  months,  of  1891  and  1892.  I 
am  also  indebted  to  Mr.  S.  H.  Scudder  for  the  identification 
of  several  Orthoptera. 

I.  The  Embryonic  Development  of  the  Locustid^. 

I.  The  Oviposition  of  Xiphidiiim  ensiferimt,  Scud. 

Xiphidmm  ensiferum,  Scudder,  a  very  common  Locustid  in 
Wisconsin  and  the  neighboring  states,  deposits  its  eggs  in  the 
silvery  napiform  galls  produced  by  Cecidoinyia  gitaphaloides 
(and  perhaps  allied  species)  on  the  low  willows  that  abound  in 
the  marshy  lands  and  along  small  water  courses.  I  have  found 
the  insect  ovipositing  from  the  middle  of  August  to  the  middle 
of  September.  It  thrusts  its  ensate  ovipositor  in  between  the 
imbricated  scales  of  the  gall  and  places  its  eggs  singly  or  in  a 
more  or  less  even  row  with  their  long  axes  directed  like  the 
long  axis  of  the  gall.  The  eggs  are  completely  concealed  by 
the  scales,  the  overlapping  edges  of  which  spring  back  to  their 
original  positions  as  soon  as  the  ovipositor  is  withdrawn. 
The  number  of  eggs  deposited  in  a  gall  varies  greatly:  some¬ 
times  but  two  or  three  will  be  found;  more  frequently  from 
fifty  to  one  hundred;  in  one  small  gall  I  counted  170  and  I 
have  opened  a  few  which  contained  more.  Sometimes  as 
many  as  ten  eggs  will  be  found  under  a  single  scale;  when 
this  is  the  case,  the  eggs  adhere  to  one  another  and  are 
more  or  less  irregularly  arranged,  as  if  two  or  three  insects 
had  in  succession  oviposited  in  the  same  place. 

The  Cecidomyia  galls  vary  considerably  in  shape:  some  are 
long  and  more  or  less  fusiform,  others  are  spheroidal.  In  the 
former  variety  the  scales  are  pointed  and  flat,  while  in  the 
latter  they  are  rounded  and  have  their  median  concave  por¬ 
tions  less  closely  applied  to  the  convex  surfaces  of  the  scales 


4 


WHEELER. 


[VOL.  VIII. 


which  they  overlap.  These  differences  materially  affect  the 
eggs,  for  many  of  those  thrust  in  between  the  closely 
appressed  scales  of  the  spindle-shaped  galls  are  so  much  flat¬ 
tened  as  to  be  incapable  of  developing;  on  the  other  hand  the 
eggs  deposited  in  the  more  spacious  interstices  of  the  globular 
galls  are  usually  in  no  wise  injured.  The  two  forms  of  gall  do 
not  always  occur  in  the  same  locality  and  may  be  the  produc¬ 
tions  of  two  distinct  species  of  Cecidomyia  or  of  one  species 
on  different  willows.  The  Locustids,  however,  seem  to  show 
no  preference  for  the  globular  galls. 

The  galls  of  Cecidomyia,  being  essentially  stem-galls,  do 
not  drop  to  the  ground  in  the  autumn  like  the  various  leaf- 
galls  on  the  willows,  but  persist  through  several  seasons.  Al¬ 
though  the  insects  are  not  averse  to  ovipositing  in  the  fresh 
galls,  they  nevertheless  seem  to  prefer  these  blackened  and 
weather-beaten  specimens,  probably  because  their  scales  are 
more  easily  forced  apart. 

I  have  called  attention  to  the  fact  (’90^)  that  X.  ensifenim 
departs  widely  in  its  habits  of  oviposition  from  its  congeners, 
several  of  which  are  known  to  lay  their  eggs  in  the  pith  of  easily 
penetrated  twigs,  like  the  species  of  the  allied  genus  Orcheli- 
immi.  X.  eiisiferum  has  evidently  found  it  of  great  advantage 
to  make  use  of  the  galls  so  abundant  in  its  native  haunts.  So 
recent  may  be  the  acquisition  of  this  habit,  that  on  further 
investigation  some  females  may,  perhaps,  even  now  be  found 
to  have  a  tendency  to  oviposit,  like  Conocephalns  ensiger,  be¬ 
tween  the  root-leaves  and  stems  of  plants,  or  even  in  the  plant 
tissues.  It  still  occasionally  happens  that  the  eggs  are  run 
through  or  into  the  tissues  of  the  gall-scales,  and  not  loosely 
deposited.  The  fact  that  the  insects  have  not  yet  learned  to 
distinguish  the  kind  of  gall  best  adapted  to  their  purposes, 
lends  some  support  to  the  view  that  it  is  not  so  very  long 
since  X.  e7isiferum  agreed  with  its  congeners  in  habits  of  ovi¬ 
position. ^ 

1  In  the  vicinity  of  Worcester,  Mass.,  I  found  galls  very  similar  to  those  formed 
on  the  Wisconsin  willows.  They  contained  a  few  slender  yellow  eggs,  smaller 
than  those  of  X.  ensiferum.  As  this  species  does  not  occur  in  New  England  I 
conclude  that  these  eggs  were  probably  deposited  by  the  very  common  X,  fascia- 
turn,  De  Geer. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY, 


5 


I 


2.  The  Formation  of  the  Embryo  and  its  Backward  Passage 

Through  the  Yolk. 

a.  SURFACE  CHANGES. 

The  sub-opaque,  cream-colored  egg  of  Xiphidium  is  elongate  - 
oval,  3-5  mm.  long  and  i  mm.  broad  through  its  middle.  One 
of  its  poles  is  distinctly  more  attenuate  than  the  other,  and 
there  is  a  faint  curvature  in  the  polar  axis  which  causes  one 
side  of  the  egg  to  be  distinctly  convex  and  the  other  distinctly 
concave.  The  broader  pole  is  the  posterior,  and  is  the  first 
to  leave  the  vagina  during  oviposition;  the  attenuate  pole  is, 
therefore,  the  anterior.  In  the  galls  the  eggs  stand  with  their 
attenuate  poles  pointing  upwards.  The  convex  face  of  the  egg 
is  the  ventral,  the  concave  face  the  dorsal  region.  Inasmuch 
as  the  egg  undergoes  no  change  in  shape  during  development, 
it  is  easy  to  orient  the  embryo  in  its  different  stages.  This  is 
of  considerable  importance,  as  will  appear  from  the  sequel. 

The  yolk  is  pale  yellow  and  very  similar  in  constitution  to 
the  yolk  of  other  Orthopteran  eggs.  It  is  enclosed  by  a  thin 
leathery  chorion  which  suddenly  becomes  transparent  on 
immersion  in  alcohol.  When  dry  it  is  white,  and  the  creamy 
color  of  the  egg  is  due  to  the  yellow  yolk  shining  through.  As 
in  Blatta,  the  chorion  is  the  only  envelope  of  the  freshly  laid 
egg  ;  what  I  described  in  a  former  paper  (’90t>)  as  the  vitelline 
membrane  is  in  reality  comparable  to  a  ‘‘ Blastodermhaut  ”  as  I 
shall  point  out. 

The  chorion  varies  somewhat  in  thickness  at  different  points 
in  the  egg,  being  ii/x  towards  the  middle  and  igfx  at  the 
poles.  It  is  quite  elastic  and  when  cut  curls  in  at  the  edges. 
Its  inner  surface  is  very  smooth,  while  outwardly  it  is  covered 
with  round  or  oval  projections  which  measure  about  3.7/a  in 
diameter.  They  are  flattened  at  their  summits  and  are  placed 
so  closely  together  that  only  narrow  channels  run  between 
them  and  give  the  chorion  the  appearance  of  being  covered 
with  a  fine  net  of  nearly  uniform  meshes.  On  closer  examina¬ 
tion  it  is  seen  that  the  projections  are  arranged  in  hexagonal 
groups.  These  are  very  distinct  at  either  pole  but  fade  away 


6 


WHEELER. 


[VOL.  VIII. 


on  the  median  portions  of  the  egg  till  they  become  very  diffi¬ 
cult  to  resolve.  They  evidently  coincide  with  the  areas  covered 
by  the  polygonal  cells  of  the  follicular  epithelium. 

No  traces  of  micropyles  could  be  found.  Their  absence  in 
Xiphidium  is  of  interest,  since  Leuckart  (’55)  long  since 
described  and  figured  them  in  several  European  Locustidae 
{Meco7ievia,  Decticiis^  Loctista,  Ephippigera) .  In  these  genera 
they  consist  of  funnel-like  perforations  on  the  ventral  surface 
of  the  chorion  either  near  the  anterior  pole  or  nearer  the 
middle  of  the  egg. 

The  preblastodermic  stages  were  not  studied.  They  prob¬ 
ably  resemble  the  corresponding  stages  of  Blatta^  of  which  I 
have  given  a  detailed  account  in  a  former  paper  (’89). 

When  fully  formed  the  Xiphidium  blastoderm,  like  that  of 
Blatta^  consists  of  a  thin  sheet  of  cells,  that  have  in  part 
reached  the  surface  from  the  interior  of  the  egg,  and  are  in 
part  derived  from  these  centrifugal  cells  by  tangential  division 
after  their  arrival  at  the  surface.  Numerous  cells  —  the  future 
vitellophags  —  are  to  be  found  at  different  points  in  the  yolk. 
Whether  they  are  derived  from  the  incompleted  blastoderm 
by  centripetal  division,  or  are  inhibited  before  reaching  the 
surface,  my  limited  observations  will  not  permit  me  to  decide. 

The  cells  forming  the  blastoderm  are  polygonal,  much  flat¬ 
tened  and  of  uniform  size  and  distribution.  Those  on  the 
center  of  the  convex,  or  ventral  face  of  the  egg  soon  begin  to 
change  their  dimensions;  from  being  broad  and  flat,  they 
become  more  nearly  cubical,  their  lenticular  nuclei  again 
assuming  the  spherical  or  oval  shape  which  they  had  in 
preblastodermic  stages.  These  changes  take  place  over  a 
limited  and  somewhat  oval  area  and  result  in  the  formation  of 
the  ventral  plate.  The  few  eggs  that  I  have  been  able  to  find 
in  the  very  first  stages  after  the  completion  of  the  blastoderm 
leave  me  in  some  doubt  as  to  the  exact  process  whereby  the 
embryo  is  established.  I  am  satisfied,  however,  that  the 
thickening  and  narrowing  of  the  individual  blastodermic  cells 
does  not  take  place  simultaneously  o’^er  the  whole  ventral 
plate  area,  but  that  there  appear,  as  in  the  crustacean  egg  {eg. 
Astacus,  Homarus),  several  discrete  centres  about  which  the 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY, 


7 


cells  are  at  first  more  closely  aggregated.  The  spaces  between 
these  centres  are  subsequently  filled  in  by  tangential  cell- 
divisions.  Of  such  centres  I  can  distinguish  four:  two  of  them, 
the  precursors  of  the  procephalic  lobes,  are  paired,  while  the 
other  two  form  respectively  the  growing  caudal  end  of  the 
ventral  plate  and  what  I  shall  call  the  indusium.^  The  indusial 
centre,  which  does  not  make  its  appearance  till  a  short  time 
after  the  other  centres  are  formed,  does  not  join  the  body 
of  the  embryo  till  after  the  spaces  between  the  procephalic 
and  caudal  centres  are  filled  in.  This  is  distinctly  seen  in  Fig. 
I  (Stage  A)  where  the  somewhat  T-shaped  embryo  is  already 
established  and  distinctly  marked  off,  at  least  posteriorly,  from 
the  undifferentiated  blastoderm.  The  nuclei  of  the  blastoderm 
are  as  yet  no  larger  than  the  nuclei  of  the  ventral  plate. 
Numerous  caryokinetic  figures  in  all  parts  of  the  embryo  bear 
witness  to  active  cell  proliferation.  No  such  figures  were  to 
be  seen  in  the  extra-embryonal  blastoderm  during  and  after 
this  stage.  The  ventral  plate  including  the  indusium  is 
scarcely  a  fifth  as  long  as  the  egg,  being  much  smaller  in  pro¬ 
portion  to  the  size  of  the  yolk  than  in  some  other  Orthoptera 
{Blatta,  Gryllotalpa). 

The  blastopore  is  seen  in  the  stage  figured  as  a  very  narrow 
but  distinct  groove  extending  from  the  oral  region  to  the 
caudal  end  of  the  embryo,  where  it  bifurcates  before  its 
termination.  The  infolded  cells  give  rise  to  the  mesoderm 
and  also,  I  believe,  to  the  entoderm. 

In  Xiphidhim  the  three  folds  that  form  the  amnion  and 
serosa  arise  like  their  homologues  in  Blatta.  The  first  appears 
as  a  crescentic  duplication  surrounding  the  caudal  end;  thence 
it  grows  forward  and  after  enveloping  the  whole  postoral 
portion  of  the  embryo  coalesces  with  the  two  head-folds,  each 
of  which  arises  from  the  edge  of  a  procephalic  lobe.  The  pro¬ 
gress  of  the  anal  fold  is  shown  in  Fig.  2  (Stage  B)  PI.  I. 
Although  agreeing  in  its  main  features  with  what  has  been 
described  for  most  insect  embryos,  the  process  of  envelope- 

^  In  a  preliminary  note  (’90°)  this  structure  was  called  the  praeoral  plate 
(Praoralplatte).  Many  reasons  have  led  me  to  abandon  this  term  together  with 
others  referring  to  the  parts  of  the  organ  in  its  subsequent  development. 


8 


WHEELER. 


[VOL.  VIII. 


formation  in  Xiphidmm^  is,  nevertheless,  peculiar  in  two 
respects:  first,  the  envelopes  are  so  closely  applied  to  the 
germ-band  that  in  surface  view  their  advancing  edges  can 
be  detected  only  with  difficulty,  though  they  may  be  distinctly 
seen  in  sections;  second,  the  point  of  closure  of  the  envelopes 
is  situated  further  forward  on  the  head  than  in  Blatta^  Hydro- 
pJiiltis,  DorypJiora,  etc.  This  I  infer  from  an  embryo,  which  I 
figure  (Fig.  15.  PI.  II.)  Here  the  cells  and  nuclei  of  the 
amnion  and  serosa  have  become  much  larger  than  the  cells 
and  nuclei  of  the  embryo.  The  edges  of  the  folds  are 
unusually  distinct  and  enclose  a  circular  space  through  which 
the  oral  and  praeoral  regions  are  clearly  visible.  On  the 
median  anterior  edge  of  the  head  the  amnion  and  serosa  are 
completely  interrupted.  In  no  other  insects  have  I  found  the 
envelopes  lacking  on  the  anterior  edge  of  the  head  in  so  late 
a  stage.  This  fact  is  probably  significant  when  taken  in  con¬ 
nection  with  changes  about  to  occur  in  front  of  the  head. 

The  wide  procephalic  lobes  are  succeeded  by  the  strap-shaped 
body  In  this  a  number  of  segments  have  made  their  appear¬ 
ance.  These  are  in  order  from  before  backwards:  the  mandib¬ 
ular  ((ind.  s),  the  first  maxillary  {inx.  ji),  the  second  maxillary, 
(p7ix.  s'^)  the  three  thoracic  (/.  s^-p.  sP),  and  the  first  abdominal 
{a.  j*!).  Further  back  lies  a  small  segment  which  is  incom¬ 
pletely  constricted  off  from  the  first  abdominal  and  which  I 
take  to  be  the  proliferating  terminal  segment,  or  telson.  The 
seven  segments  depicted  in  the  figure  are  undoubtedly  de¬ 
finitive  segments.  The  manner  of  their  appearance  will  be 
clear  from  a  glance  at  Fig.  I.  In  A  the  ligulate  part  of  the 
germ-band  is  seen  to  be  faintly  constricted  at  its  base  into  two 
segments  with  indications  of  a  third.  In  B,  a  slightly  later 
stage,  four  definitive  postoral  segments  are  present,  but  a 
portion  of  the  germ-band  still  remains  unsegmented.  This  is, 
however,  soon  broken  up  into  segments  and  we  reach  the  stage 
in  Fig.  15,  PI.  II.  It  will  be  observed  that  the  embryos  in 
Fig.  I  are  in  many  respects  older  than  that  in  Fig.  15,  PI.  II. 
The  antennae  have  made  their  appearance  and  the  amniose- 
rosal  fold  has  closed  completely.  These  embryos  prove 
several  points:  —  first,  that  the  wave  of  metameric  segmen- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


9 


tation  passes  from  before  backwards  dividing  the  germ-band 
into  7  or  8  segments;  second,  that  these  segments  are  the 
definitive  segments  and  not  macrosomites,  or  complexes  of 
definitive  segments;  and  third,  that  there  is  considerable  varia¬ 
tion  in  the  time  when  segmentation  sets  in.  To  these  points 
I  may  add  a  fourth ;  segmentation  appears  first  in  the  ectoderm 
and  only  somewhat  later  in  the  mesoderm. 


A  and  B.  Isolated  embryos  of  Xiphidiuni  in  successive  stages  of  metameriza- 
tion.  ind.,  indusium  ;  pel.,  procephalic  lobe  ;  st.,  stomodaeum  ;  at.,  antenna  ;  md.  s., 
mandibular  segment ;  nix.  s'-,  first  ;  mx.  second,  maxillary  segment ;  p. 
prothoracic  segment. 


The  indusium  (Fig.  15,/.  t?.)  is  still  only  a  rounded  thickening 
of  the  blastoderm.  Its  small  deep  cells  are  continuous  through 
a  zone  of  larger  cells  with  the  relatively  very  large  and  flat 
elements  of  the  primitive  cell-layer.  Two  broad  and  flat 
commissures  appear  to  connect  the  organ  with  the  procephalic 
lobes.  Thus  a  small  space  containing  a  few  larger  cells  is 
enclosed  between  the  indusium  and  the  head  of  the  embryo. 
This  space  (j),  seen  as  a  clear  spot  in  surface  view,  lies  at  the 
breach  in  the  envelopes.  In  many  embryos  the  indusium  is 


lO 


WHEELER. 


[VOL.  VIII. 


united  with  the  head  of  the  embryo  (Fig.  I,  A  and  B)  before 
the  stage  of  Fig.  15  and  soon  after  this  stage  is,  I  believe, 
normally  united  with  it.  This  union  is  probably  purely 
mechanical  —  the  organ  remaining  at  its  place  of  origin  on 
the  surface  of  the  yolk,  while  the  embryo  lengthens  till  its 
head  unites  with  the  posterior  end  of  the  organ.  This  union 
is  of  brief  duration  as  is  seen  in  Fig.  3  (Stage  C). 

During  this  stage  the  caudal  tip  of  the  embryo  shows  a 
tendency  to  bury  itself  in  the  yolk.  The  amnion  and  serosa, 
hitherto  closely  applied  to  each  other,  now  separate  at 
the  caudal  end,  where,  as  I  have  said,  they  first  arose  as  a 
crescentic  fold.  Soon  the  tendency  to  enter  the  yolk  becomes 
more  pronounced  so  that  the  tail  curls  back  and  leaves  the 
ventral  face  of  the  egg.  Meanwhile  the  remainder  of  the 
embryo  moves  down  the  ventral  face  a  short  distance,  thus 
pushing  its  tail  still  further  into  the  yolk  and  causing  the 
separation  of  the  envelopes  to  advance  still  further  headwards. 
The  indusium  does  not  accompany  the  embryo  in  this  move¬ 
ment,  but  remains  nearly  or  quite  stationary  ;  consequently 
the  head  gradually  separates  from  the  organ  till  it  is  connected 
only  by  means  of  a  slender  band  of  cells  in  the  median 
line.  (Fig.  3  and  Fig.  16.)  This  link  soon  ruptures  and  the 
indusium  is  set  adrift  from  the  embryo,  or,  more  precisely, 
the  embryo  is  set  adrift  from  the  indusium.  (Fig.  4,  Stage  D.) 
In  profile  the  embryo  now  resembles  the  small  letter  j, — the 
dot  being  supplied  by  the  isolated  indusium. 

Important  changes  begin  to  affect  the  indusium  during  or 
more  frequently  just  after  its  separation  from  the  embryo. 
The  closely  packed  cells  at  the  periphery,  as  indicated  by  their 
nuclei,  begin  to  arrange  themselves  radially  (Fig.  16).  Some 
of  the  large  nuclei  of  the  serosa  may  be  seen  encroaching  on 
the  edges  of  the  disk  from  all  sides,  leaving  only  the  median 
portion  free.  Sections  show  that  the  organ  is  now  forming  an 
amnion  like  that  of  an  embryo.  In  the  middle  of  the  disk 
appear  several  shrunken  but  distinctly  defined  nuclei  which  are 
proved  by  focusing  to  be  confined  to  the  surface  of  the  organ. ^ 

1  Only  four  of  these  peculiar  bodies  are  represented  in  the  figure  {n7i) ;  there 
were  several  others  in  the  same  preparation,  but  for  the  sake  of  clearness  I  have 
omitted  them  in  the  drawing. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  II 

The  serosal  fold  continues  to  advance  from  all  sides  till  the 
organ  is  entirely  covered.  Viewed  from  its  ventral  surface 
the  egg  now  has  the  appearance  of  Fig.  4  (Stage  D).  Here 
the  indusium  is  cordate  in  outline  and  somewhat  larger  than 
usual.  Of  the  abdomen  only  the  two  basal  segments  still 
remain  on  the  ventral  face  of  the  egg  ;  the  remaining  seg¬ 
ments  curl  back  into  the  yolk. 

During  this  and  the  two  preceding  stages  the  cephalic  and 
thoracic  appendages  have  become  distinctly  established  as 
rounded  lateral  outgrowth  of  their  respective  segments.  The 
antennae  (at)  originate  as  lobular  outgrowth  from  the  posterior 
edges  of  the  procephalic  lobes.  They  are  distinctly  postoral 
in  origin.  The  margins  of  the  triangular  oral  orifice  are  some¬ 
what  swollen;  the  anterior  edge,  where  the  labrum  is  about 
to  appear,  is  cleft  in  the  median  line  (Fig.  16).  The  three 
thoracic  segments  are  very  slightly  or  no  broader  than  the  two 
maxillary  segments.  The  appendages  of  these  five  segments 
are  also  alike  in  size,  shape,  and  position.  In  very  early  stages 
of  other  insect  embryos,  even  before  the  amnion  and  serosa 
are  fully  formed,  the  thoracic  become  broader  than  the  maxil¬ 
lary  segments,  and  the  legs,  as  soon  as  they  appear,  may  be 
readily  distinguished  from  the  two  pairs  of  maxillae  by  their 
greater  size  and  prominence.  The  Locustid  embryo,  there¬ 
fore,  has  even  a  stronger  tendency  to  revert  to  annelid-like  or 
myriopod-like  ancestors  than  is  apparent  in  any  of  the  other 
insects  whose  ontogenies  have  been  investigated. 

The  mandibular  segment  of  XipJiiditim  like  that  of  other 
insects,  is  somewhat  retarded  in  its  development.  Between 
this  and  the  antennary  segment  careful  study  of  sections  and 
surface  preparations  reveals  the  presence  of  another  segment, 
shown  very  distinctly  in  outline  in  Fig.  \6  (^tc.  s}j.  This  is 
no  other  than  what  I  have  called  the  intercalary  segment  in 
Dorypliora.  It  is  the  tritocerebrum  of  Viallanes  (’90^’  ’90^). 

The  embryo  continues  to  move  back  into  the  yolk,  fol¬ 
lowing  the  curved  path  established  by  the  inflexion  of  the 
posterior  segments  till  its  tail  is  finally  arrested  by  striking  the 
serosa  on  the  dorsal  surface.  At  this  time  the  embryo  has  the 
form  of  an  arc  subtending  the  dorsoventral  diameter  of  the  egg. 


12 


WHEELER. 


[VOL.  VIII. 


Returning  to  consider  the  indusium,  we  find  that  it  begins 
to  increase  in  size  before  the  embryo’s  head  leaves  the 
ventral  face.  The  organ  stains  much  less  deeply,  and  even  in 
surface  view  its  expansion  may  be  seen  to  be  due  to  a  flatten¬ 
ing  of  its  component  cells.  In  Fig.  5  (Stage  E)  is  represented 
an  embryo  merged  in  the  yolk  up  to  the  first  maxillary  seg¬ 
ment.  The  indusium  extends  around  on  either  side  nearly 
to  the  middle  of  the  lateral  face  of  the  egg.  Either  the  tran¬ 
sition  of  the  embryo  takes  place  rapidly  or  the  organ  changes 
very  gradually,  for  the  latter  is  in  about  the  same  stage  after 
the  embryo  has  become  established  on  the  dorsal  surface. 
The  manner  in  which  the  expansion  of  the  indusium  is 
brought  about  will  be  apparent  when  I  come  to  describe  its 
structure  in  sections. 

b.  THE  INDUSIUM  IN  SECTION. 

As  will  be  seen  from  the  preceding  account,  the  indusium 
is  simply  a  circular  thickening  of  the  blastoderm,  situated 
in  the  median  line,  between  and  a  little  in  front  of  the 
procephalic  lobes.  It  does  not  arise  as  a  part  of  the  ventral 
plate  but  as  a  separate  centre  which  is  at  first  merely  a 
cluster  of  blastoderm  cells  that  have  changed  from  the  pave¬ 
ment  to  the  cubical  or  columnar  type.  This  centre  is  further 
increased  in  breadth  and  thickness  by  caryokinesis.  In  the 
earliest  stages  examined,  sections  of  the  organ  show  the  same 
cell-structure  as  sections  of  the  procephalic  lobes. 

Median  longitudinal  sections  of  the  embryo  in  Stage  C  are 
interesting  as  showing  the  relations  of  the  indusium  to  the 
embryo  and  its  envelopes.  I  reproduce  such  a  section  in  Fig. 
21,  PI.  III.  Here  the  organ  (/.  0)  appears  as  a  large  flattened 
cell-aggregate  somewhat  thinner  in  the  centre  than  nearer  its 
periphery.  Owing  to  the  shape  of  the  mass,  the  median  cells, 
as  indicated  by  their  nuclei,  are  arranged  with  their  long  axes 
perpendicular  to  the  flat  outer  surface  of  the  organ,  while  the 
cells  of  the  thickened  lateral  portions  become  gradually  more 
oblique  till  those  on  the  extreme  periphery  assume  the  same 
position  as  the  serosa  cells  {s).  The  nuclei  are  most  frequently 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


13 


situated  at  the  inner  ends  of  the  cells  so  that  masses  of 
enucleate  protoplasm  are  left  at  the  surface.  Posteriorly  the 
organ  is  linked  to  the  embryo  by  means  of  a  few  flattened 
cells.  In  the  section  two  of  these  cells  are  seen  at  z  differing 
in  no  wise  from  the  serosal  elements  (i-.)  in  front  and  on  either 
side  of  the  organ  ;  the  upper  cell  passes  directly  into  the  serosa 
covering  the  embryo,  while  the  lower  abuts  on  the  cells  that 
form  the  transition  from  the  ectoderm  to  the  amnion.  The 
ectodermal  layer  of  the  embryo  {ec^j  is  nearly  as  thick  as  the 
indusium  and  of  similar  cytological  structure.  The  begin¬ 
ning  of  the  stomodaeal  invagination  is  shown  at  0. 

The  next  section  figured  (Fig.  17  PI.  II)  is  from  an  indusium 
in  a  somewhat  younger  stage  than  that  represented  in  surface 
view  in  Fig.  2.  Being  transverse  the  section  shows  an  evenly 
convex  outer  surface,  continuous  with  the  surface  of  the  serosa 
(i-.)  enveloping  the  yolk.  The  cell-contours  are  still  visible 
and  show  that  the  cells  constituting  the  median  portion  of  the 
organ  are  polygonal.  The  nuclei  of  these  elements  are  spherical 
or  oval  and  contain  one,  or  more  rarely,  two  nucleoli  besides  the 
usual  chromosomes.  In  the  peripheral  ring-shaped  thickening 
the  cells  (<^.)  are  larger  and  pyramidal  or  fusiform  in  outline, 
while  their  nuclei  differ  in  no  wise  from  the  nuclei  of  the 
median  cells.  The  serosal  cells  stain  more  deeply  than  the 
cells  of  the  organ,  as  may  be  seen  at  where  a  single  cell 
overlaps  the  edge  of  the  disk.  This  depth  of  color  is  appar¬ 
ently  purely  optical,  being  due  to  the  greater  size  and  flatness 
of  the  serosal  nuclei.  The  walls  of  both  the  small  polygonal 
and  larger  pyramidal  elements  fade  away  towards  the  surface, 
where  the  bodies  of  the  different  cells  become  confluent 
to  form  a  homogeneous  mass. 

In  this  surface-mass  of  protoplasm  which  takes  the  normal 
pink  stain  in  borax  carmine,  are  to  be  found  several  of  the 
peculiar  nuclei,  mentioned  above  as  distinctly  discernible  from 
the  surface  (Fig.  16).  They  differ  markedly  in  structure  and 
appearance  from  the  normal  nuclei  in  the  inner  portions  of  the 
indusium  as  will  be  seen  by  comparing  the  cells  of  Fig.  24 
with  those  in  Fig.  23,  both  of  which  figures  were  drawn  with 
a  high  power.  The  normal  cells  (Fig.  23)  have  spherical  or 


14 


WHEELER. 


[VOL.  VIII. 


oval,  evenly  rounded  nuclei  with  one  or  two  nucleoli  and  their 
chromatin  is  distributed  in  what  I  take  to  be  the  typical  resting 
reticulum.  The  caryolymph,  or  Kernsaft,  is  faintly  stainable. 
On  the  other  hand,  in  the  nuclei  of  Fig.  24  the  nuclear  wall  is 
very  irregular,  the  caryolymph  much  more  limpid  and  refrac¬ 
tive  and  the  chromatic  reticulum  has  coarser  meshes.  The 
chromatic  nodes  of  the  reticulum  are  larger  than  in  Fig.  23 
and  seem  to  be  applied  to  the  indentations  of  the  nuclear  wall. 
Nucleoli  appear  to  be  absent.  These  specialized  nuclei  also  vary 
greatly  in  size.  In  a  series  of  sections  it  is  easy  to  find  nuclei 
intermediate  between  the  two  extremes  here  described, 
being  evenly  rounded  but  with  colorless  caryolymph  and 
coarse  chromatic  reticulum.  A  cluster  of  four  such  nuclei 
is  shown  at  mi'^  Fig.  17.  These  intermediate  forms,  occurring 
as  they  usually  do,  between  the  normal  and  the  modified 
nuclei  may  be  taken  to  indicate  that  the  nuclei  of  the  extreme 
types  are  genetically  connected.  Some  of  the  normal  nuclei 
probably  leave  their  respective  cells  in  the  median  portions  of 
the  organ  and  move  up  into  the  syncytial  protoplasmic  layer, 
undergoing  the  modification  in  structure  during  their  emigra¬ 
tion.  When  they  have  reached  their  destination  they  are 
perhaps  broken  down  and  converted  into  protoplasm.  Certain 
it  is  that  later  no  traces  of  them  are  to  be  found  in  the 
indusium.  I  do  not  believe  that  I  am  here  considering  collapsed 
and  distorted  caryokinetic  figures,  as  these  delicate  structures 
are  quite  faithfully  preserved  in  eggs  killed  by  means  of  heat. 
The  distorted  nuclei  are  not  confined  to  the  indusium  but 
occur  also  in  the  ectoderm  of  the  embryo  itself. 

When  the  organ  has  reached  the  state  just  described  it 
usually  separates  from  the  head  of  the  embryo;  it  may,  how¬ 
ever,  remain  attached  for  some  time  longer.  Like  the  embryo 
it  is  now  an  isolated  body  lying  on  the  yolk;  but  unlike  the 
embryo  it  is  still  only  a  part  of  the  serosal  envelope  (which  is 
itself  only  the  extra-embryonal  portion  of  the  blastoderm).  The 
serosa  is  a  closed  sack  enveloping  the  whole  yolk  and  the 
indusium  is  simply  a  swelling  at  one  point  on  its  inner 
face.  (Fig.  II,  A.)  The  process  of  envelope  formation  which 
now  begins  in  the  indusium  is  much  less  clear  than  the  cor- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


15 


responding  process  already  completed  in  the  embryo.  From 
among. the  numerous  preparations  which  I  have  made  I  select 
for  illustration  one  (Fig.  18)  which  seems  to  show  the  process 
clearly.  In  surface  view  the  organ  would  appear  as  in  Fig.  3. 
The  spreading  of  the  serosal  cells  over  the  edges  of  the 
disk  from  all  sides  is  now  seen  to  be  due  to  a  process  of 
induplication,  or  folding.  The  circular  fold  is,  of  course, 
cut  in  two  places  in  the  median  transverse  section  figured. 
It  advances  in  such  a  manner  as  to  leave  the  outer  face  of 
the  indusium  evenly  rounded  and  undisturbed,  the  upper  sur¬ 
face  of  the  fold  usually  forming  a  continuous  line  both  with 
the  outer  surface  of  the  serosa  and  with  the  median  still 
uncovered  portion  of  the  organ.  The  fold  continues  to  advance 
from  all  sides  till  the  layers  of  which  it  consists  meet  and 
become  confluent  in  essentially  the  same  manner  as  the  folds 
that  form  the  amniotic  and  serosal  layers  over  the  embryo 
proper.  We  now  have  three  layers  of  cells.  (Fig.  19.)  The 
outermost  layer,  j,  is  the  serosa  which  has  everywhere  the  same 
structure  and  evenly  envelops  the  whole  egg,  having  been 
separated  first  from  the  embryo  and  now  by  a  similar  process 
also  from  the  indusium  (Fig.  II,  B).  The  innermost  layer 
consists  of  the  unchanged  greater  portion  of  the  organ.  The 
median  layer,  to  judge  from  its  component  cells,  seems  to 
be  derived  exclusively  from  cells  of  the  original  body  of  the 
organ  and  not  from  the  serosa.  This  layer  is,  therefore,  like 
the  amnion  of  the  embryo  proper,  structurally  more  closely 
related  to  the  body  it  envelops  than  to  the  serosa.  Fig.  18 
favors  this  conclusion,  which  presupposes  that  only  the  outer 
half  of  the  circular  fold  is  derived  from  the  serosa,  for  in  this 
section  the  lower  and  thicker  layer  of  the  fold  on  either  side 
certainly  consists  of  cells  derived  from  the  body  of  the  organ. 
Even  before  the  layers  are  fully  formed  the  edges  of  the  two¬ 
layered  organ  are  sharp  and  somewhat  irregular  (Fig.  18),  not 
rounded  like  the  edges  of  the  embryo  when  its  amnion  is  com¬ 
pleted.  The  whole  organ  still  has  essentially  the  same  form 
that  it  had  in  the  stage  represented  in  Fig.  17. 

It  will  be  convenient  to  name  the  different  layers  of  cells, 
thus  far  distinguished.  For  the  amnion  of  the  embryo  proper 


i6 


WHEELER. 


[VOL.  VIII. 


I  shall  retain  the  old  name;  the  corresponding  envelope  of  the 
indusium  and  the  body  of  the  organ  will  be  designated  as  the 
outer  and  inner  indusium  respectively. 

In  by  far  the  greater  number  of  cases  the  process  of 


Fig.  II. 


Diagrams  illustrating  the  movements  and  envelopes  of  the  Xiphidium  embryo. 
A,  after  the  closure  of  the  amnioserosal  folds  ;  B,  during  the  embryo’s  passage  to 
the  dorsal  surface  ;  C,  just  after  the  straightening  of  the  embryo  on  the  dorsal 
surface,  ind.,  indusium  —  afterwards  forming  the  inner,  and  ind'^,  the  outer 
indusium  ;  ch.,  chorion  ;  sr.,  serosa  ;  am.,  amnion  ;  gb.,  germ-band  ;  v.,  yolk  ;  bl.  c., 
Blastodermhaut. 


envelope  formation  over  the  indusium  is  much  obscured  by 
rapid  slurring.  In  fact  the  whole  process  has  frequently  the 
appearance  of  being  due  rather  to  a  shifting  and  migration 
of  cells  than  to  the  formation  of  true  folds.  The  cells  of  the 
serosa  seem  to  creep  over  the  disk  while  the  cells  forming  the 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY, 


17 


edge  of  the  organ  itself  appear  to  creep  along  under  and  a  little 
in  the  rear  of  the  advancing  serosal  elements.  I  cannot  here 
go  into  greater  detail  without  unduly  increasing  the  number  of 
my  figures.  Nor  is  it  necessary,  since  it  will,  I  believe,  be 


Fig.  II. 


Diagrams  illustrating  the  movements  and  envelopes  of  the  Xiphidmm  embryo. 
Z?,  the  stage  of  the  shortened  embryo  on  the  dorsal  yolk  ;  E,  embryo  returning  to 
the  ventral  surface  ;  Z%  embryo  nearly  ready  to  hatch,  ch.,  chorion  ;  bl.  c.,  Blasto- 
dermhaut  ;  sr.,  serosa  ;  ind^,  outer  indusium  ;  ind'^,  inner  indusium  ;  md^  am., 
inner  indusium  and  amnion  fused  ;  am.,  amnion  ;  ind'^  c.,  cuticle  of  the  inner 
indusium  ;  ind'^  s.,  granular  secretion  of  inner  indusium  ;  am.  s.,  amniotic 
secretion  ;  v.,  yolk  ;  cl.,  columella  ;  gb.,  germ-band. 

acceded  that  the  process  briefly  described  in  the  above 
paragraph,  though  now  occurring  in  comparatively  few 
embryos,  is  very  probably  the  more  primitive  process,  whereas 
the  slurring  observed  in  so  many  cases  is  to  be  attributed  to  an 
unquestionably  rudimental  condition  of  the  organ. 


i8 


WHEELER. 


[VoL.  VIII. 


By  the  time  the  folds  have  closed  over  the  indusium  the 
abdomen  of  the  embryo  has  sunk  into^  the  yolk  to  a  con¬ 
siderable  extent,  presenting  in  surface  view  the  appearance 
of  Fig.  4.  The  organ  seems  to  undergo  no  further  change  till 
the  embryo  has  almost  left  the  ventral  face  of  the  egg.  Then, 
as  we  have  seen,  it  begins  to  increase  by  spreading.  An  early 
stage  in  this  process  is  shown  in  section  in  Fig.  20.  No  change 
is  perceptible  in  the  serosa,  which  is  now  independent  of  the 
organ  ;  the  outer  indusium  (am'^')  is  much  attenuated,  as  may 
be  seen  by  comparison  with  Fig.  19.  Its  cells  have  assumed 
the  same  shape  and  dimensions  as  those  of  the  superjacent 
serosa  ;  only  along  the  edges  of  the  disk,  where  the  outer 
becomes  continous  with  the  inner  indusium,  or  body  of  the 
organ,  do  the  cells  still  retain  their  original  shapes.  In 
the  body  of  the  organ  the  cells  are  arranged  in  two  irreg¬ 
ular  rows,  whereas  in  the  previous  stage  (Fig.  20)  there  were 
three.  This  diminution  in  the  number  of  cell-rows  is  the 
result  of  horizontal  spreading,  a  process  which  also  accounts 
for  the  stretching  of  the  outer  indusium  as  indicated  by  the 
flatness  of  its  cells.  At  nn  is  seen  one  of  the  large  modified 
nuclei,  which  has  persisted  unusually  late. 

In  Fig.  22  I  give  a  section  of  the  indusium  seen  in  sur¬ 
face  view  in  Fig.  5.  The  spreading  of  the  cells  has  pro¬ 
gressed  till  the  organ  lies  like  a  saddle  on  the  ventral  face  of 
the  egg,  covering  nearly  half  of  its  circumference.  The  serosal 
layer  {s)  is,  of  course,  unaffected.  The  outer  indusium  {ani^) 
is  stretched  to  such  an  extent  that  its  cells  are  united  only  by 
an  exceedingly  thin  and  in  many  places,  almost  imperceptible 
layer  of  protoplasm.  The  inner  indusium  now  consists  of  a 
single  row  of  cells,  instead  of  two  rows  as  in  the  preceding 
stage.  It  is  in  about  the  same  state  of  tension  as  the  outer 
layer  in  Fig.  19. 

3.  The  Development  of  the  Embryo  from  the  Time  of  its 
Reaching  the  Dorsal  Yolk  to  RevoliUion. 

In  the  foregoing  paragraphs  the  development  of  the  embryo 
was  traced  to  Stage  E,  when  the  germ-band  hangs  festoon-like 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


19 


in  the  yolk  with  its  cephalic  amnion  applied  to  the  ventral 
serosa  and  the  amnion  overlying  its  terminal  abdominal  seg¬ 
ments  applied  to  the  serosa  covering  the  dorsal  yolk.  No 
sooner  has  the  caudal  end  become  fixed  than  the  head  is 
detached  from  the  ventral  face  of  the  egg  and  the  embryo 
swings  back,  straightens  out,  and  becomes  applied  full  length 
to  the  dorsal  serosa.  The  movements  whereby  this  condition 
is  attained  resemble  the  movements  of  a  leech  in  passing  from 
one  side  of  a  test-tube  to  the  opposite  surface;  holding  fast  to 
the  glass  by  means  of  the  oral  sucker,  the  tail  is  stretched  out 
till  it  reaches  the  opposite  surface,  when  the  anterior  end  is 
loosened  and  the  body  drawn  over.  There  is,  however,  a 
marked  difference  between  the  embryo  and  the  leech  since 
the  body  of  the  former  is  not  contracted  during  its  transition. 

Fig.  5  represents  a  rather  rare  condition  in  that  the  pro- 
cephalic  lobes  lie  at  the  same  level  and  are  symmetrically  dis¬ 
posed  with  respect  to  the  long  axis  of  the  egg.  More  frequently 
the  germ-band  is  twisted  during  its  transition  so  that  one  of  the 
procephalic  lobes  reaches  further  forward  than  the  other  on 
the  surface  of  the  yolk.  Sometimes  it  is  the  left  lobe  which 
extends  further  forward  but  more  frequently  it  is  the  right. 
The  twist  in  the  germ-band  occurs  in  the  thoracic  or 
abdominal  region,  more  often  in  the  former,  the  abdomen  being 
nearly  straight.  I  take  this  twisting  of  the  embryonic  axis  to 
indicate  that  the  germ-band  executes  a  screw-like  movement 
while  penetrating  the  yolk,  and  I  believe  it  to  be  perfectly 
normal,  having  observed  it  in  the  majority  of  embryos.  Traces 
of  this  twisting  are  clearly  discernible  even  in  embryos  which 
have  almost  straightened  on  the  dorsal  surface. 

As  a  consequence  of  the  passage  of  the  embryo  through  the 
yolk  in  the  manner  above  described,  the  germ-band  has  shifted 
its  position  from  the  median  convex  ventral  to  the  median 
concave  dorsal  surface  of  the  yolk,  so  that  it  is  now  reversed: 
originally  its  head  pointed  to  the  tapering  anterior  pole,  now 
it  lies  with  its  head  directed  towards  the  blunt  posterior  pole 
of  the  egg.  The  amnion,  of  course,  accompanies  and  remains 
in  close  contact  with  the  ventral  surface  of  the  embryo  during 
all  this  time. 


20 


WHEELER. 


[VoL.  VIIL 


During  or  more  frequently  at  the  close  of  the  embryo’s  migra¬ 
tion  the  primary  serosa  secretes  from  its  whole  outer  surface 
a  thin  chitinous  cuticle.  In  my  preliminary  notes  (’90^’  ’90°) 
I  wrongly  designated  this  cuticle  as  the  vitelline  membrane,  an 
error  which  is,  to  a  certain  extent,  pardonable,  inasmuch  as 
the  layer  in  question  is  structurally  exactly  like  the  vitelline 
membranes  of  other  insects.  But  it  certainly  cannot  be 
homologized  with  these  membranes  since  it  is  secreted  during 
a  comparatively  advanced  stage  by  an  embryonic  cell-layer, 
the  serosa,  and  not  by  the  surface  protoplasm  of  the  un¬ 
segmented  egg. 

As  soon  as  the  embryo  has  taken  up  its  position  on  the 
dorsal  surface,  the  yolk  segments  ;  each  vitellophag  appro¬ 
priating  as  many  of  the  yolk-bodies  as  the  radiating  filaments 
of  its  cytoplasm  can  hold  together  and  fashion  into  a  rounded 
mass.  Apparently  the  process  is  delayed  in  order  that  the 
passage  of  the  embryo  through  the  yolk  may  be  facilitated,  for 
obviously  the  embryo  will  move  more  easily  over  a  prescribed 
path  through  a  mass  of  small  mobile  particles  than  between 
large  masses  formed  by  the  aggregation  of  such  particles.  The 
yolk-masses,  at  first  very  distinctly  marked,  soon  fuse  with 
one  another  so  that  their  boundaries  can  be  traced  only  by 
reference  to  their  centres,  which  coincide  with  the  nuclei  of 
the  vitellophags. 

After  leaving  the  ventral  face  of  the  egg  the  embryo  in¬ 
creases  greatly  in  length.  Just  before  burying  its  tail  in 
the  yolk  and  while  still  completely  on  the  ventral  surface  it 
measured  only  .7  mm.;  now  it  measures  1.7  mm.  This  in¬ 
crease  in  length,  as  will  be  inferred  from  the  foregoing  descrip¬ 
tion,  is  due  to  two  causes  :  an  intercalation  of  new  segments 
in  front  of  the  anal  plate  to  complete  the  abdomen,  and  a 
stretching  of  the  segments  thus  arising. 

A  glance  at  Fig.  6,  which  represents  an  embryo  in  the  stage 
of  its  greatest  elongation  on  the  dorsal  surface,  shows  that 
many  important  changes  have  taken  place  since  it  left  the 
ventral  surface.  The  cephalic  and  thoracic  appendages  have 
assumed  a  more  definite  character.  The  labrum  {lb})  has  sud¬ 
denly  appeared,  the  first  and  second  maxillae  mx^')  have 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


21 


each  become  trilobed,  while  the  metathoracic  leg  (/3)  already 
exhibits  unmistakable  traces  of  its  characteristic  thickening  in 
the  larva  and  imago.  The  pleuropodia  (//.  stand  out  clearly 

from  the  edges  of  the  first  abdominal  segment.  Shining 
through  the  stretched  ectodermal  layer  of  the  abdominal  seg¬ 
ments  may  be  seen  the  paired  mesodermal  somites  {coe}j^  or 
mesomeres.  The  anal  plate  with  its  pair  of  cerci  (<:<:.  > 

and  the  anus  are  definitely  established.  A  faint  neural  furrow 
runs  from  the  mouth  to  the  anus,  and  in  the  thoracic  region 
faint  metameric  indications  of  the  ganglia  are  apparent.  All 
these  important  changes  have  taken  place  within  the  yolk  during 
the  transition  of  the  embryo.  This  renders  their  study  on 
hardened  material  very  difficult,  for  although  the  embryo  may 
be  dissected  away  from  the  yolk,  it  is  so  much  curved  that  it 
can  be  mounted  only  in  pieces,  and  the  yolk  is  at  this  period 
so  difficult  to  cut  that  only  fragmentary  series  of  sections  can 
be  obtained. 

One  of  the  most  interesting  changes  undergone  while  the 
embryo  is  still  in  the  yolk  is  the  appearance  of  the  labrum.  In 
Fig.  6  (Stage  F)  the  labrum  is  a  distinctly  unpaired  circular 
appendage.  But  that  it  has  a  paired  origin  I  infer  from  a 
transverse  section,  part  of  which  is  represented  in  Fig.  35. 
This  passes  just  in  front  of  the  mouth  of  an  embryo  but  little 
older  than  Stage  E.  The  appendage  {Ib^j  is  here  seen  to  be 
distinctly  bilobed  although  it  does  not  yet  project  beyond  the 
general  level  of  the  head.  This  bilateral  condition  is  speedily 
slurred  over  and  the  organ  grows  into  an  unpaired  and  in 
most  embryos  perfectly  circular  disk  overhanging  the  mouth. 
Very  rarely,  as  in  Fig.  7  it  may  show  traces  of  its  paired  origin 
even  during  later  stages. 

Let  us  return  to  the  indusium  which  we  left  as  a  thin 
round  plate  gradually  spreading  over  the  yolk  just  beneath  the 
ventral  serosa.  The  outlines  of  this  plate  are  not  always  cir¬ 
cular  but  exhibit  traces  of  lobulation  (Fig.  5).  The  spreading 
is  at  first  uniform  along  its  whole  circumference  so  that  the 
organ  soon  assumes  the  shape  of  a  circular  scroll  clasping  the 
egg.  Its  lateral  edges  approximate  on  the  dorsal  surface  just 
over  the  ventral  face  of  the  embryo  but  are  temporarily  arrested 


22 


WHEELER. 


[VOL.  VIII. 


in  their  growth  before  they  unite.  The  anterior  and  posterior 
edges,  however,  continue  to  advance  without  interruption,  so 
that  the  disk  if  spread  out  on  a  plane  surface  would  in  its  suc¬ 
cessive  stages  represent  a  series  of  ellipses  with  constant  short 
axis  but  continually  increasing  longitudinal  axis.  In  this 
manner  the  disk  grows  towards  either  pole  while  envelop¬ 
ing  the  egg  laterally.  The  edges  of  the  organ  continue  to 
approximate  on  the  dorsal  surface  but  stop  growing  just  before 
they  meet.  Hence,  when  the  egg  is  viewed  from  the  dorsal 
surface  a  long,  narrow  slit  is  seen  extending  nearly  its  entire 
length  and  separating  the  dorsal  edges  of  the  organ.  It  is  not 
till  the  anterior  and  posterior  edges  have  nearly  or  quite 
reached  their  respective  poles  that  this  slit  closes  with  the 
fusion  of  the  edges  of  the  organ.  The  raphe  is  at  first  so 
weak  that  the  edges  may  be  broken  apart  by  slight  pressure 
with  the  needles,  but  it  soon  becomes  permanent  and  the  egg 
is  now  completely  enveloped  by  two  further  membranes  —  the' 
inner  and  outer  indusia.  Before  the  fusion  of  these  two  mem¬ 
branes  the  amnion  of  the  embryo  was  in  contact  with  the 
serosa  but  now  that  the  edges  of  the  indusia  have  worked 
their  way  in  between  the  serosa  and  amnion,  the  latter  comes 
to  lie  in  contact  with  the  inner  indusium.  Henceforth  the 
serosa  is  excluded  from  taking  any  part  in  the  development 
of  the  embryo;  both  its  position  and  function  are  now  usurped 
by  the  inner  indusium. 

One  is  enabled  to  follow  the  different  stages  in  the  progress 
of  the  indusium,  from  its  disk-like  condition  on  the  ventral 
yolk  to  the  complete  union  of  its  dorsad-growing  edges,  by 
means  of  a  peculiar  secretion  of  its  inner  layer.  This  is  a 
brownish  or  blackish  granular  substance,  probably  some  urate, 
which  appears  to  be  secreted  by  all  the  cells  of  the  inner 
indusium  and  which  gives  the  organ  the  appearance  of  a  large 
brown  blotch  in  a  stage  a  little  older  than  E.  At  first  pale  and 
hardly  perceptible,  this  spot  gradually  deepens  in  color  till  its 
advancing  edges  become  distinctly  outlined  on  the  underly¬ 
ing  yolk.  A  clear  idea  of  the  closure  of  the  edges  may  be 
obtained  from  Fig.  HI,  A-C.  The  dark  granular  secretion 
is  shown  in  Fig.  6  at  envl. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  23 

Soon  after  the  union  of  the  edges  of  the  outer  and  inner 
indusial  layers  a  chitinious  cuticle  is  secreted  by  the  outer 
surface  of  the  latter.  This  cuticle  is  thicker  and  seems  to  be 
of  a  deeper  hue  than  the  cuticle  secreted  by  the  serosa.  It 


Fig.  III. 


Two  stages  in  the  spreading  of  the  indusium.  A,  lateral  view  of  egg  just  after 
the  arrival  of  the  embryo  on  the  dorsal  yolk  ;  lateral  view  of  the  egg  with  the 
indusium  nearly  reaching  the  poles  ;  C,  same  egg  seen  from  the  dorsal  surface. 

definitely  excludes  the  outer  indusium  from  any  further  share 
in  the  development  of  the  embryo.  Even  in  Stage  E,  this  cell- 
layer  was  reduced  to  an  exceedingly  thin  membrane.  ( PI. 
Ill,  Fig.  22,  am.^)  It  seems  to  fuse  with  the  serosa  and  to 
retain  a  connection  with  the  inner  indusium  only  at  the  ex- 


24 


WHEELER. 


[VOL.  VIII. 


treme  anterior  pole  of  the  egg.  I  confess,  however,  that  my 
observations  on  this  envelope  are  rather  unsatisfactory. 

After  the  completion  of  the  processes  described  in  the 
preceding  paragraphs  we  may  distinguish  several  envelopes 
in  a  median  transverse  section  of  the  egg.  Passing  from  with¬ 
out  inwards  we  have  (i)  the  chorion,  (2)  the  Blastodermhaut- 
like  cuticle  secreted  by  the  serosa,  (3)  the  serosa,  (4)  the 
outer  indusium,  (5)  the  layer  of  dark,  granular  secretion,  (6) 
the  cuticle  secreted  by  the  inner  indusium,  (7)  the  inner 
indusium  and  (8)  the  amnion.  While  envelopes  1-7  invest 
the  whole  egg,  layer  8,  the  amnion,  covers  only  the  embryo. 

The  general  development  of  the  embryo  has  been  traced  to 
Stage  F,  when  it  lies  as  a  straight  and  attenuated  body  on  the 
dorsal  yolk  with  its  head  directed  towards  the  caudal  and  its 
tail  towards  the  cephalic  pole  of  the  egg. 

Like  all  other  insects  that  have  a  stage  during  which  the 
body  is  greatly  elongated  (Coleoptera,  Diptera,  Lepidoptera) 
Xiphidium  passes  into  a  series  of  stages  during  which  the 
germ-band  is  gradually  shortened.  The  shortening  is  accom¬ 
panied  by  a  broadening  of  all  the  segments,  a  growth  of  the 
appendages,  and  very  important  internal  changes.  The  com¬ 
pletion  of  this  process  is  reached  in  Stage  G  (Fig.  7).  Besides 
a  greater  development  of  the  appendages  seen  in  Stage  F,  Fig. 
7  also  shows  that  the  abdominal  appendages  have  appeared. 
Of  these  there  are  nine  pairs,  exclusive  of  the  pleuropodia 
and  cerci,  so  that  in  Xiphidium,  just  as  in  Blatta  and  many 
other  insects,  every  segment  of  the  abdomen  bears  a  pair  of 
appendages.  Starting  with  the  basal  segment  there  are  eight 
pairs  of  stigmata.  These  are  not  all  seen  in  the  figure.  Just 
back  of  each  pair  of  tracheal  invaginations  appears  a  second 
pair  of  ingrowths  —  the  metastigmatic  depressions  —  seen  as 
small  white  spots  just  outside  the  appendages,  near  the  pos¬ 
terior  edges  of  their  respective  segments.  They  are  in  line 
(homostichous)  with  the  tracheal  invaginations  which  occupy 
corresponding  positions  near  the  anterior  edges  of  their 
respective  segments. 

The  ventral  flexure  of  the  abdomen  constitutes  another  very 
important  difference  between  Stages  G  and  F.  In  Xiphidmm 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


25 


this  flexure  always  takes  place  between  the  7th  and  8th  seg¬ 
ments  and  is  brought  about  during  the  shortening  of  the 
embryo.  It  is  essentially  the  same  flexure  which  is  found  in 
Blatta  and  in  Decapod  Crustacea. 

In  Stage  G  the  antennae  have  increased  to  nearly  one-third 
the  length  of  the  embryo.  The  procephalic  lobes  on  which  the 
segmentation  of  the  brain  is  plainly  visible,  have  developed 
greatly.  The  appendages,  instead  of  projecting  laterally,  as 
they  do  in  the  younger  embryo,  are  folded  over  the  ventral 
surface  of  the  germ-band.  The  nerve  cord  is  distinctly 
marked  out.  (See  abdominal  region.  Fig.  7.) 

It  is  in  this  stage,  or  one  but  slightly  more  advanced,  that 
the  embryo  passes  the  winter.  Cleavage  and  the  succeeding 
stages  up  to  F  are  passed  within  a  month  after  oviposition  — 
during  the  warm  days  of  August  and  September.  But  even 
should  October  and  November  be  mild  and  sunny,  development 
seems  to  have  come  to  a  temporary  standstill  on  reaching 
Stage  G.  Among  the  hundreds  of  embryos  which  I  collected 
during  three  succeeding  autumns,  I  did  not  find  one  that  had 
passed  far  beyond  this  stage.  Nevertheless  if  kept  in  a  warm, 
moist  atmosphere  during  winter,  a  certain  number  of  eggs 
will  continue  their  development  almost  to  hatching. 

Before  passing  on  to  later  stages  in  the  development  I  will 
here  give  a  brief  account  of  some  anomalies  in  the  development 
of  the  indusium. 

4.  Variations  in  the  Development  of  the  Indusium. 

In  the  preceding  pages  I  have  described  what  I  take  to 
be  the  normal  development  of  the  indusium  of  Xiphidium. 
A  considerable  number  of  embryos  (about  100),  being  nearly 
one  half  of  the  total  number  examined  for  the  stages  thus  far 
described,  deviated  more  or  less  widely  in  so  far  as  the  in¬ 
dusium  was  concerned  from  what  I  consider  the  normal  type  of 
development.  Unfortunately  I  did  not  discover  the  organ  till 
it  was  too  late  in  the  season  to  obtain  a  large  supply  of  material 
in  the  requisite  stages,  so  that  the  variations  here  briefly 
noticed  probably  represent  only  a  small  fraction  of  those 


26 


WHEELER. 


[VOL.  VIII. 


observable  in  a  large  number  of  eggs.  The  variations  may  be 
tabulated  thus  :  — 

1.  Variations  in  size.  Normally  the  indusium  is  of  the 
same  size  as  one  of  the  procephalic  lobes  (.2  mm.  in  diameter) 
so  that  the  head  of  the  embryo  resembles  a  clover  leaf  as  long 
as  the  organ  is  attached  to  it.  When  the  chorion  is  removed 
the  organ  may  be  distinctly  seen  with  the  unaided  eye  as  a 
milk-white  spot  on  the  translucent  yolk.  Occasionally,  how¬ 
ever,  embryos  will  be  found  in  which  it  is  less  than  .1  mm.  in 
diameter,  and  all  variations  between  this  and  the  normal  size 
ipay  be  observed. 

2.  Variations  from  the  typical  circular  form.  These  varia¬ 
tions  are  very  numerous  and  may  be  regarded  as  belonging  to 
two  classes.  In  one  class  the  indusium  is  rounded  in  outline, 
while  in  the  other  it  is  ragged  and  more  or  less  irregular. 
To  the  first  class  may  be  assigned  the  oval,  cordate  and 
multilobulate  varieties  not  infrequently  observed;  to  the  second 
belong  a  number  of  irregularly  stellate  and  rhizopod-like  forms. 
In  one  of  my  preparations,  midway  between  the  two  classes, 
the  indusium  is  evenly  rounded  anteriorly  and  ragged  poste¬ 
riorly  along  that  portion  of  its  periphery  which  has  just  broken 
away  from  the  head  of  the  embryo. 

3.  There  is  a  variation  in  the  time  at  which  the  organ 
is  set  free  from  the  head.  This  cannot  be  proved  directly 
by  observation  of  the  organ  itself,  for  it  usually  does  not 
begin  to  form  the  circular  fold  till  after  its  isolation,  but 
differences  in  the  embryo,  especially  in  the  prominence  of  the 
segments  and  appendages,  show  that  the  organ  remains  at¬ 
tached  to  the  head  in  some  cases  longer  than  in  others. 

4.  Variations  in  the  development  of  the  circular  fold.  These 
variations,  alluded  to  above,  are  characterized  by  a  greater  or 
less  distinctness  in  the  folds  that  give  rise  to  the  inner  and 
outer  layers.  All  shades  in  the  process  may  be  found  between 
the  distinct  and  comparatively  rare  method  described  and 
figured  (Fig.  3),  and  the  more  frequent  and  obscurer  method 
whereby  the  three  layers  are  formed  by  a  shifting  of  the 
individual  cells. 

5.  Variations  in  number.  I  have  twice  observed  two  indusia 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


27 


in  the  same  egg.  In  the  first  case  the  embryo  itself  was  in 
every  way  normal,  and  the  first  indusium  of  the  normal  size 
and  shape,  and  in  the  usual  position.  The  second,  somewhat 
smaller,  though  regularly  circular  organ,  was  situated  in  front 
of  the  first  and  a  little  to  the  right  of  the  median  line.  The 
distance  between  the  two  organs  was  about  double  the  distance 
between  the  first  organ  and  the  head  of  the  embryo.  The 
outlines  of  the  second  or  more  anterior  organ  were  less  definite 
than  those  of  the  first.  The  amnion  and  serosa  had  formed 
over  the  embryo,  but  neither  of  the  indusia  showed  as  yet  any 
tendency  to  form  envelopes.  Whether  these  two  organs  were 
derived  from  the  division  of  one  original  praeoral  cluster  of  cells, 
or  were  originally  established  as  two  separate  centres  on  the 
blastoderm,  I  am  unable  to  decide.  The  latter  method  would 
seem  to  be  the  more  probable. 

The  other  case  is  somewhat  singular.  The  first  indusium 
was  normal  in  size  and  position,  but  irregularly  heptagonal 
in  outline.  The  second,  situated  a  short  distance  to  the 
side  of  the  right  procephalic  lobe,  was  not  more  than  a  third 
the  size  of  the  first  Organ  and  quite  regularly  quadrangular. 
The  embryo  itself  was  normal  and  covered  with  the  amnion 
and  serosa.  The  envelopes  had  also  formed  over  the  two 
organs,  which  in  this  case  also  probably  originated  from  two 
discrete  centres  in  the  blastoderm.  The  smaller  organ  had 
probably  never  been  attached  to  the  head  of  the  embryo. 

5.  The  Revohttion  of  the  Embryo 

During  the  first  warm  days  of  spring  the  Xiphidiufu  embryo 
resumes  its  development.  This  is  characterized  for  some  time 
by  a  growth  of  the  germ-band  in  breadth  and  length  and  a 
lengthening  of  the  appendages.  The  body  of  the  embryo, 
which  in  Stages  F  and  G  was  much  narrower  than  the  egg 
now  becomes  almost  as  broad  so  that  its  pleural  edges  embrace 
the  yolk.  This  increase  in  size  brings  the  head  somewhat 
nearer  the  lower  pole,  and  there  soon  sets  in  a  decided  move¬ 
ment  of  the  whole  body  in  this  direction.  When  the  head  has 
almost  reached  the  lower  pole,  the  amnion  covering  the  face 


28 


WHEELER. 


[VOL.  VIII. 


of  the  cephalic  end  fuses  with  the  overlying  inner  indusium. 
A  rent  appears  in  this  fused  portion  of  the  envelopes  and 
through  it  the  head  is  soon  seen  protruding.  Gradually  more 
of  the  body  is  pushed  through  the  orifice,  first  the  mouth  parts, 
then  the  thoracic  legs  and  finally  the  abdominal  segments,  till 
the  whole  embryo  comes  to  lie  free  on  the  surface  of  the  yolk 
in  the  space  between  the  inner  indusium  and  its  cuticle.  The 
amnion  and  inner  indusium,  which  during  the  evagination  of 
the  embryo  have  remained  united  at  the  edges  of  the  rent 
are  folded  over  the  pleural  region  of  the  embryo  onto  the  yolk. 
The  two  envelopes  now  form  but  a  single  layer  enclosing  the 
yolk  like  a  bag.  The  inner  indusium  is  united  to  the  edges  of 
the  amnion  and  these  in  turn  are  united  to  the  pleural  edges  of 
the  embryo,  with  the  ectoderm  of  which  the  amniotic  cells  are 
continuous.  The  small  size  of  the  amniotic  cells  as  compared 
with  the  huge  flattened  elements  of  the  inner  indusium  enables 
one  readily  to  distinguish  the  limits  of  the  two  envelopes. 

During  its  evagination  from  the  cavity  of  the  amnion  the 
embryo  gradually  passes  around  the  lower  pole  of  the  egg 
head  first  and  begins  to  ascend  the  convex  ventral  surface.  An 
embryo  freed  from  all  its  envelopes  except  the  two  that  take 
part  in  revolution  is  represented  in  Fig.  8,  in  the  very  act  of 
turning  the  lower  pole.  The  amnion  and  inner  indusium  are 
folded  back  over  the  yolk,  the  former  {ant)  characterized  by  its 
small  rounded  nuclei,  the  latter  {sr^j  by  its  large  flat  elements. 
The  line  of  juncture  of  the  amnion  with  the  body  of  the 
embryo  is  marked  by  a  denser  aggregation  of  nuclei.  The 
ventral  flexure  still  persists  on  the  dorsal  surface. 

The  cavity  of  the  amnion  contains  a  quantity  of  serum-like 
liquid,  which  during  the  evagination  of  the  embryo  is  poured 
into  the  space  separating  the  inner  indusium  from  its  cuticle. 
This  liquid  collecting  at  the  lower  pole,  may  function  as  a 
lubricant  and  cushion,  and  thus  facilitate  the  movements  of  the 
germ-band.  In  hardened  specimens  it  is  found  as  a  gran¬ 
ular  magma  enveloping  the  appendages.  It  is  not  shown  in 
Fig.  8. 

In  many  respects  the  embryo  in  Stage  H  has  advanced  con¬ 
siderably  beyond  that  represented  in  Fig.  7.  In  the  head,  the 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


29 


eye  is  distinctly  marked  out  and  its  cells  are  arranging  them¬ 
selves  to  form  the  ommatidia,  as  is  evident  from  the  regular 
series  of  pale  dots.  The  labrum,  now  considerably  enlarged, 
is  spade-shaped  in  ventral  aspect.  The  antennae  have  grown 
in  length,  and  the  saltatory  legs  (/3)  are  assuming  their  defin¬ 
itive  characters.  The  large  tapering  pleuropodia  stand  out 
prominently  on  the  first  abdominal  segment.  Near  the  bases 
of  the  legs  the  thoracic  stigmata  are  distinctly  seen.  They 
had  made  their  appearance  in  Stage  G,  but  for  obvious  reasons 
could  not  be  shown  in  the  figure. 

The  anterior  end  of  the  embryo  continues  to  move  up  the 
ventral  surface  of  the  egg,  straightening  out  as  it  rises. 
Finally  the  flexed  terminal  segments  of  the  abdomen  are 
again  bent  back  to  their  original  position  in  line  with  the 
rest  of  the  body.  Since  their  flexure  these  segments  (the  8th- 
iith)  have  been  the  only  portion  of  the  body  provided  with  a 
completed  dorsal  wall  {vide  Fig.  7).  After  the  bending  back 
of  the  abdominal  tip  its  segments  still  retain  a  certain  inde¬ 
pendence  and  make  no  attempt  to  embrace  the  yolk  of  the 
posterior  pole  as  do  the  segments  in  front  of  them.  It  is  for 
this  reason  that  the  abdomen  presents  a  constriction  just  in 
front  of  the  eighth  segment.  This  constriction  is  especially 
noticeable  in  profile  view. 

The  turning  of  the  lower  pole  of  the  egg  seems  to  take  place 
very  rapidly  compared  with  other  equally  important  processes 
of  development,  such  as  the  passage  of  the  embryo  through 
the  yolk.  I  infer  this  from  the  relative  scarcity  of  embryos  in 
the  act  of  returning  to  the  ventral  surface.  I  have,  however, 
succeeded  in  finding  all  the  stages  in  the  process  of  revolution, 
and  feel  quite  as  confident  of  having  correctly  interpreted  my 
preparations  as  if  I  had  studied  the  living  egg. 

6.  The  Stages  Intervening  between  Revolution  and 

Hatching. 

Fig.  9  represents  an  embryo  that  has  just  straightened  out 
on  the  ventral  surface  of  the  yolk,  which  the  reader  may 
imagine  as  extending  up  beyond  the  head  to  nearly  twice  the 


30 


WHEELER. 


[VoL.  VIIL 


length  of  the  embryo  and  terminating  in  the  pointed  anterior 
pole.  A  comparison  of  Figs.  8  and  9  shows  that,  although 
the  former  embryo  has  completed  its  revolution,  it  is  neverthe¬ 
less  in  an  earlier  stage  so  far  as  the  development  of  its  organs 
is  concerned.  This  is  particularly  noticeable  in  the  labrum, 
antennae  and  mouth  parts,  the  eyes  and  the  saltatory  legs. 
Hence  we  may  infer  that  the  time  for  turning  the  lower  pole 
is  subject  to  considerable  variation. 

In  Fig.  9  it  will  be  observed  that  many  of  the  abdominal 
appendages  have  disappeared.  Pairs  are,  however,  retained  on 
the  8th  to  nth  segments  (ap^-cc,  The  pleuropodia 

are  also  still  present  though  concealed  behind  the  bases  of  the 
metathoracic  legs.  The  disappearance  of  the  appendages  on 
the  2d-8th  segments  probably  has  its  immediate  mechanical 
cause  in  the  lateral  stretching  which  characterizes  these  seg¬ 
ments  in  their  attempts  to  embrace  the  yolk. 

The  embryo  continues  its  growth  as  before  in  two  directions 
—  the  body  constantly  lengthening  and  thus  bringing  the  head 
nearer  the  pointed  anterior  pole,  while  its  lateral  walls,  envelop¬ 
ing  more  and  more  of  the  yolk,  gradually  grow  towards  each 
other  and  finally  unite  in  the  median  dorsal  line.  The  union 
begins  with  the  7th  abdominal  segment,  just  in  front  of  the  seg¬ 
ments  which  have  for  some  time  been  provided  with  a  dorsal 
wall,  and  continues  headward.  I  am  not  certain  as  to  what 
becomes  of  the  amnion  during  this  process.  Its  cells  appear 
to  take  no  part  in  the  formation  of  the  dorsal  wall,  but  very 
probably  degenerate  and  become  supplanted  by  the  cells  of  the 
advancing  ectoderm.  It  must  be  remembered  that  a  hard  and 
fast  line  cannot  be  drawn  between  the  amnion  and  the  pleural 
ectoderm  ;  the  cells  of  both  structures  passing  into  one  another 
by  insensible  gradations.  My  reasons  for  supposing  that  the 
amnion  proper  takes  no  part  in  building  up  the  embryo  are 
mainly  of  a  theoretical  nature  and  will  be  given  in  the  latter 
part  of  this  paper. 

Concerning  the  fate  of  the  inner  indusium  there  can  be  little 
doubt.  While  the  embryo  is  continually  advancing  towards 
the  cephalic  pole  and  enclosing  more  and  more  of  the  yolk  — 
this  envelope,  which,  as  above  stated,  is  characterized  by 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


31 


huge  flat  cells  and  nuclei,  is  being  as  gradually  restricted  to  a 
more  and  more  limited  yolk  surface.  In  consequence  of  this 
restriction  its  component  cells  become  broader  radially  and 
narrower  tangentially.  In  this  stage  the  envelope  functionally 
corresponds  to  the  “dorsal  organ”  of  other  insects.  It  cannot, 
however,  be  thus  designated  without  still  further  increasing 
the  number  of  heterogeneous  structures  included  under  that 
unfortunate  term,  since  the  “dorsal  organ”  of  other  insects  is 
a  thickening  of  an  envelope  represented  in  Xiphidium  by  the 
serosa. 

The  thickened  inner  indusium  is  soon  reduced  to  a  cap  of 
cells  on  the  anterior  pointed  pole  of  the  egg.  As  the  head  of 
the  embryo  advances  to  cover  more  of  this  pole,  the  envelope 
is  pushed  further  forward  and  finally  stripped  from  the  yolk 
altogether.  The  anterior  cranial  walls  then  close  over  the 
pole  and  thus  effectually  separate  the  yolk  from  the  inner 
indusium.  The  latter  is  reduced  to  a  small  conical  mass,  the 
cells  of  which  soon  show  unmistakable  signs  of  degeneration. 

Soon  after  the  embryo  has  thus  rid  itself  of  its  envelopes 
and  has  taken  into  its  mesenteron  the  whole  mass  of  yolk  not 
utilized  in  the  processes  of  development  hitherto  undergone,  a 
chitinous  cuticle  is  shed  from  its  entire  surface.  This  may  be 
designated  as  the  first  larval  cuticle.  It  appears  first  on  the 
ventral  abdominal  surface  and  spreads  thence  headward  and 
dorsad.  The  progress  of  cuticularization  is  readily  traceable 
by  staining  embryos  in  this  stage,  for  the  parts  over  which  the 
cuticle  is  formed  will  not  take  the  color  ;  where  it  is  being 
deposited  the  stain  takes  faintly  and  where  it  has  not  yet 
appeared,  the  stain,  of  course,  penetrates  easily.  Ayers  (’84) 
observed  in  CEcanthus  that  the  secretion  of  the  cuticle 
began  on  the  ventral  surface  of  the  embryo  and  extended 
dorsad.  This  is  just  what  we  should  expect  from  the  fact  that 
the  dorsal  hypodermis  is  ontogenetically  a  more  recent  forma¬ 
tion  than  that  of  the  ventral  surface. 

The  first  larval  cuticle  is  about  5  /x  thick  and  consists  of 
three  layers.  The  innermost  is  apparently  homogeneous  and 
stains  deeply  in  Orth’s  lithium  carmine  while  the  middle  layer 
remains  clear  and  vitreous.  The  outer  layer  is  radially  striated 


32 


WHEELER. 


[VOL.  VIII. 


and  has  the  distinctly  yellow  tint  of  old  chitin.  Its  outer  sur¬ 
face  is  minutely  papillate.  On  the  appendages  the  cuticle  is 
much  thinner  than  it  is  on  the  trunk  and  though  it  stains  it 
does  not  show  a  differentiation  into  three  layers. 

Before  shedding  the  first  cuticle  the  hypodermis  secretes  a 
second  larval  skin  which  persists  till  after  hatching. 

In  Fig.  IV,  I  have  attempted  to  represent  semi-diagrammatic- 
ally  the  condition  of  the  envelopes  at  a  time  when  the  eyes 
begin  to  acquire  pigment.  The  chorion  {ch^i  is  much  distended 
and  the  egg  larger  and  more  resistent  to  the  touch  then  it  was 
during  the  autumn.  Passing  from  without  inward  we  first 
meet  with  the  cuticle  secreted  by  the  serosa  {sr.  c).  Then 
follows  the  serosa  itself  (sr^  to  the  inner  face  of  which  the 
remains  of  the  outer  indusium  {ind.^)  are  applied.  At  the  ex¬ 
treme  anterior  end  of  the  egg  both  these  cellular  envelopes 
appear  to  be  much  thickened  and  pass  into  a  cylindrical  pedicel 
of  granular  plasma  which  I  shall  call  the  columella  {cl).  This  in 
turn  is  continuous  with  a  conical  mass  of  cells  the  re¬ 

mains  of  the  inner  indusium  which  was  stripped  from  the  head 
in  a  preceding  stage.  Its  cells,  as  shown  in  the  figure,  are  in  an 
advanced  stage  of  disintegration.  The  cytoplasm  of  the  different 
elements  is  reduced  to  a  mass  of  granules  and  the  chromosomes 
have  become  agglomerated  into  little  spheres  floating  in  the 
clear  nuclear  plasma.  The  process  of  degeneration  is  similar  to 
that  which  I  have  described  as  occurring  in  the  ‘‘dorsal  organ” 
of  Blatta.  Between  the  mass  of  degenerating  cells  and  the 
head  of  the  embryo  lies  a  granular  coagulum  {am.  s).  This  I 
take  to  be  the  amniotic  serum  which  is  forced  up  into 
the  anterior  pole  by  the  enlarging  of  the  embryo  and  the 
consequent  decrease  in  the  space  between  the  body  walls  and 
the  chorion.  The  columella  and  the  remains  of  the  inner  indu¬ 
sium  are  held  together  and  thus  temporarily  prevented  from 
complete  disintegration  by  the  thick  cuticle  of  the  latter. 
This  cuticle  still  envelops  the  embryo  and  extends  forward  to 
the  anterior  pole  where  it  seems  to  be  attached  to  the  inner  face 
of  the  outer  indusium.  Passing  further  inward  we  next 
meet  with  the  first  larval  cuticle  {Iv.  which  has  been  shed, 
and  the  second  larval  cuticle  {Iv.  c^)y  which  is  still  in  organic 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY, 


33 


connection  with  the  hypodermis.  In  a  little  later  stage  than 
the  one  here  described  the  columella  and  the  conical  lump  of 
inner  indusial  elements  have  disintegrated,  and  can  no  longer 
be  distinguished  from  the  granular  amniotic  serum. 

The  changes  in  the  configuration  of  the  embryo  since  its 
arrival  on  the  ventral  yolk,  relate  mostly  to  the  appendages. 
At  first  the  antennae  are  of  about  the  same  thickness  as  the 


Sagittal  section  through  the  anterior  pole  of  a  Xiphidium  embryo,  with  pig¬ 
mented  eyes,  ch.,  chorion  ;  cL,  columella  ;  sr.  c.,  Blastodermhaut ;  sr.,  serosa  ; 
ind^  +  remains  of  the  inner  indusium  and  amnion  ;  md^,  outer  indusium  ; 
ind'^  j-.,  secretion  of  the  inner  indusium  ;  am.  s.,  amniotic  secretion  ;  Iv.  d,  first 
larval  cuticle  ;  Iv.  d,  second  larval  cuticle  ;  br.,  brain  ;  e.,  eye. 


legs.  The  dark  line  running  parallel  with  their  inner  edges, 
and  distinctly  marked  in  Fig.  9,  is  in  section  seen  to  be  a  meso¬ 
dermal  partition  dividing  the  cavity  of  the  appendage  into  two 
tubular  sinuses.  The  antennae  grow  directly  tailward  till  their 
tips  reach  the  femorotibial  joint  of  the  hind  legs,  when  they  di¬ 
verge  laterally,  describe  an  arc,  and  then  grow  forward.  When 
the  tips  have  reached  the  head  further  progress  is  arrested 


34 


WHEELER. 


[VOL.  VIII. 


by  the  envelopes,  but  as  the  growth  of  the  appendages  does 
not  cease,  the  arcs  surrounding  the  hind  legs  gradually  move 
tailward.  This  movement  is  arrested  just  before  the  time  for 
hatching,  when  the  antennae  have  grown  to  nearly  twice  the 
length  of  the  embryo. 

The  mouth-parts  and  thoracic  appendages  have  been  gradu¬ 
ally  assuming  their  adult  characters  in  the  meantime. 

The  pleuropodia,  as  described  in  a  former  paper  (’90®),  are 
shed  during  hatching  and  just  previous  to  that  event  may  be 
found  attached  to  the  pleural  cuticle  by  means  of  very  slender 
pedicels. 

In  the  male  the  appendages  of  the  9th  and  nth  abdom¬ 
inal  segments  persist,  the  former  as  the  stylets,  the  latter  as 
the  cerci.  In  the  female  the  cerci  also  persist  but  together 
with  them  also  the  pairs  on  the  8th,  9th  and  loth  segments 
(Figs.  9  and  10  —  op'^  (ap^)  —  op3  (ap^°).  These  are  converted 
into  the  gonapophyses. 

Apart  from  the  eyes  little  pigment  is  developed  in  the  hypo- 
dermis  during  embryonic  life,  unless  we  regard  as  such  the 
brown  granular  secretion  of  the  inner  indusium. 

A  number  of  eggs  kept  in  the  house  the  greater  part  of  the 
winter  hatched  May  I5th-i8th,  but  I  am  inclined  to  believe 
that  out  of  doors  the  regular  time  for  hatching  is  later,  prob¬ 
ably  not  till  the  end  of  May.  Xiphidium  fas  datum  apparently 
does  not  hatch  till  early  in  June,  since  I  found  larvae  of  this 
species  on  Naushon  Island  June  9,  which  could  not  have  been 
more  than  a  few  days  old.  Inasmuch  as  the  imagines  of  Xiphi- 
ditim  ensiferum  oviposit  on  the  average  about  Sept,  ist,  the 
whole  postembryonic  development  cannot  occupy  more  than 
three  months.  As  this  Locustid  is  monogoneutic,  nine 
months  is  therefore  required  for  embryonic  development. 
Even  if  we  deduct  the  period  of  quiescence  due  to  cold 
weather,  it  will  still  be  apparent  that  the  embryonic  stages 
must  succeed  one  another  very  slowly  in  Xiphidium  as  com¬ 
pared  with  those  of  other  Ametabola  {e.g.  Blatta),  not  to 
mention  the  Metabola. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


7.  The  Development  of  Orchelimnm  vulgare. 

This  Locustid  oviposits  like  many  of  the  smaller  members 
of  the  family  in  the  pith  of  dead  plants.  I  found  the  eggs  in 
Ohio  during  the  last  days  of  September  in  the  stems  of  the 
wild  lettuce  {Lactuca  canadensis),  so  common  along  the  edges 
of  fields  and  thickets,  and  in  the  petioles  of  the  common  elder 
{SambuctLS  canadensis).  Oviposition  probably  takes  place  in 
the  beginning  or  towards  the  middle  of  September.  In  the 
case  of  Lactuca  and  a  few  other  plants  which  I  did  not  identify, 
the  insects  had  invariably  selected  for  oviposition  the  main 
stem  of  the  flower-panicles.  From  base  to  apex  this  portion 
of  the  stem  was  punctured  at  intervals,  and  a  single  egg 
thrust  into  the  pith  a  short  distance  above  each  orifice.  It 
is  an  easy  matter  to  recognize  the  punctures  by  the  little  tufts 
which  the  insect  evidently  gnaws  from  the  woody  fibre,  before 
inserting  its  scimeter-shaped  ovipositor.  Great  care  must  be 
taken  in  splitting  the  stem,  so  as  not  to  tear  or  cut  the  eggs 
which  adhere  very  firmly  to  the  pith. 

The  eggs  are  larger  than  those  of  Xiphidiimi  ensifencm, 
being  fully  6.- 6.2  5  mm.  long.  In  shape  they  are  very  similar 
to  Xiphidiimi  eggs  except  that  the  sides  are  compressed. 
In  the  fresh  state  they  are  smooth  and  opaque,  and  of  a 
pale  drab  or  bluish  tint.  In  this  respect,  as  also  in  the 
flattening  of  their  lateral  faces,  they  form  a  transition  to  the 
eggs  of  our  larger  Locustidae,  e.  g.  Cyrtophyllus  concavus, 
Amblycorypha  uhlerii  and  Microcentrum  retinei'vis}  The 
chorion  is  not  readily  wetted  with  water,  but  like  that  of 
the  Xiphidium  egg,  immediately  becomes  transparent  when 
immersed  in  alcohol.  The  outer  envelope  is  then  seen  to  have 
a  yellow  tint,  deepening  into  brown  at  the  poles. 

As  would  be  expected  from  its  close  systematic  affinity  the 
embryonic  development  of  Orchelimum  does  not  differ  much 
from  that  of  Xiphidium.  I  have  not  seen  all  the  stages,  nor 
have  I,  as  yet,  sectioned  any  of  my  material,  but  the  stages 
which  I  have  examined  are  essentially  the  same  as  those 

1  For  a  description  of  the  eggs  of  these  species  see  an  article  on  Orthoptera,  by 
Prof.'C.  V.  Riley,  in  the  Standard  NaUiral  History,  Vol.  II.  pp.  188-189. 


I 


36  WHEELER,  [VoL.  VIII. 

described  in  XipJiidmm.  The  embryo  of  Orchelimnm  passes 
through  the  yolk  in  the  same  manner  as  the  Xiphiditim 
embryo,  shortens  on  the  dorsal  yolk,  then  grows  apace,  moves 
around  the  lower  pole  and  finally  begins  the  yolk-enveloping 
process  on  the  ventral  surface  of  the  egg  in  the  same  way 
as  the  XipJiidmm  embryo.  It  also  develops  an  indusium 
which  is  set  free  from  the  head  and  spreads  over  the  yolk 
while  the  embryo  is  passing  through  it  backwards.  In  OrcJiel- 
imum  the  inner  indusial  layer  also  secretes  a  brownish  pigment¬ 
like  substance  which  enables  one  to  follow  its  movements  as  it 
gradually  covers  more  and  more  of  the  yolk.  A  clear  slit  is 
likewise  left  on  the  dorsal  surface  between  the  folds  of  the 
organ.  But  in  the  time  of  closure  of  this  slit  OrcJielimuin 
differs  from  XipJiidhiin.  In  the  latter  insect  we  found  that 
the  slit  closed  soon  after  the  embryo  had  straightened  on  the 
dorsal  yolk,  before  it  had  shortened  very  decidedly.  In 
Ordielimum  the  closure  is  considerably  delayed.  The  embryo 
shortens,  then  grows  in  length  and  breadth,  passing  beyond 
Stage  G  of  Xiphidium  and  its  head  nearly  reaches  the  lower 
pole  before  the  two  folds  of  the  indusium  meet  and  fuse. 
Frequently  in  this  stage,  when  the  embryo  is  about  to  revolve, 
the  polar  ends  of  the  slit  are  still  open,  the  membranes  having 
fused  over  the  embryo.  In  a  little  later  stage,  however,  the 
indusium  has  completely  enveloped  the  yolk. 

II.  Remarks  on  Gastrulation  in  the  Orthoptera. 

Although  many  important  observations  have  of  late  been 
contributed  to  the  embryology  of  the  Insecta,  our  knowledge  of 
the  formation  of  the  germ-layers  in  the  Orthoptera  cannot  be 
said  to  have  made  any  signal  advance.  As  late  as  1889  so  few 
forms  of  this  order  had  been  studied  that  I  felt  justified  in 
expressing  some  doubt  as  to  whether  their  mesentoderm  was 
formed  in  the  same  manner  as  in  the  higher  Metabola  (Cole- 
optera,  Diptera,  Lepidoptera).  My  doubts  were  confirmed  by  a 
study  of  Blatta,  when  I  failed  to  find  the  oral  formative  centre 
of  the  entoderm  (’89^).^ 

1  We  need  not  go  far  to  seek  the  reasons  for  this  gap  in  our  comparative 
studies.  The  eggs  of  the  Orthoptera  are  almost  without  exception  extremely 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


Bruce  (’86)  appears  to  have  been  the  first  to  describe  the 
origin  of  the  mesentoderm  from  a  median  ingrowth  of  the 
germ-band  in  the  Orthoptera.  The  species  which  he  studied, 
is,  I  have  every  reason  to  believe,  Stagniomantis  Carolina. 
His  description  is  very  meagre  and  his  figures  are  unsatis¬ 
factory. 

More  convincing  are  Graber’s  figures  and  description  of 
mesentoderm  formation  in  Stenobothriis  ‘variabilis  (’88,  PI. 
XIV,  Fig.  II ;  PI.  XV,  Fig.  13).  His  Fig.  ii  shows  that 
there  is  in  the  median  line  a  distinct  infolding  of  the  ventral 
plate  cells  —  a  true  invagination.  In  a  more  recent  paper  (’9o), 
the  account  is  briefly  repeated  without  any  important  additions. 

In  his  recent  study  of  the  embryogeny  of  Blatta  germanicay 
Cholodkowsky  (’91®)  gives  an  account  of  the  formation  of  the 
germ-layers  more  in  harmony  with  what  we  know  of  the  process 
in  the  Coleoptera  than  the  account  which  I  gave.  But  he  has 
not  come  to  any  definite  conclusion  respecting  the  formation  of 
the  entoderm,  and  although  he  maintains  that  there  is  a  distinct 
blastoporic  groove  running  the  length  of  the  germ-band,  he 
does  not  figure  it  in  surface  view,  and  most  of  his  sections 
betray  such  an  amount  of  distortion  in  his  preparations  that 
one  may  hesitate  to  regard  the  slight  depressions  in  his  figures 
(Figs.  7,  8,  10,  etc.)  as  indicating  invagination.  Nevertheless 
I  believe  from  renewed  study  of  the  Orthoptera,  that  Cholod¬ 
kowsky  is  correct  in  deriving  the  mesoderm  from  a  median 
proliferation  of  the  primitively  one-layered  germ-band,  and  the 
entoderm  from  two  formative  centres  —  one  in  the  oral  and 
one  in  the  anal  region. 

In  Xiphiditmiy  soon  after  its  first  appearance,  the  blastoporic 
depression,  when  seen  from  the  surface  (Fig.  i),  is  a  straight 

refractory  from  a  technical  point  of  view.  The  cells  of  the  embryo  are  often 
smaller  and  less  distinct  than  they  are  in  the  Metabola.  Moreover,  the  great 
quantity  of  yolk  and  its  singular  brittleness  in  hardened  specimens  renders 
paraffin  sectioning  most  unsatisfactory,  and  rather  than  incur  the  great  expend¬ 
iture  of  time  which  working  with  celloidin  involves,  the  student  gladly  selects 
some  Coleopteran  or  Dipteran  egg  which  is  all  that  can  be  demanded  from  a 
purely  technical  point  of  view.  Nevertheless  the  Orthoptera  constitute,  by  com¬ 
mon  consent,  one  of  the  most  primitive  orders  of  the  Insecta  ;  their  eggs  are 
large  and  may  be  readily  procured  in  great  numbers ;  their  development  is  so 
gradual  that  all  the  requisite  stages  may  be  obtained  without  the  least  difficulty. 


38 


WHEELER. 


[VoL.  VIIL 


groove  extending  nearly  the  entire  length  of  the  germ-band 
and  dividing  it  into  two  symmetrical  halves.  Anteriorly  the 
groove  is  rounded  and  seems  to  end  rather  abruptly,  but 
posteriorly  it  bifurcates,  each  of  the  two  grooves  thus  arising 
being  continued  for  a  short  distance  to  either  side  till  they 
gradually  fade  away.  There  can  be  no  doubt,  it  seems  to  me, 
that  the  bifurcated  termination  of  the  blastopore  is  the  homo- 
logue  of  the  similar  structure  first  figured  by  me  in  DorypJiora 
(^89,  PI.  XVIII,  Fig.  71;  PI.  XIX,  Fig.  73)  and  subsequently 
seen  by  Graber  (’9o)  in  the  corresponding  stages  of  Lina 
tremidcE  (PI.  II,  Figs.  25  and  27).  More  recently  Cholodkowsky 
has  observed  a  similar  widening  of  the  blastopore  in  Blatta. 
Fie  attempts  to  identify  it  with  the  posterior  depressions  of 
Graber’s  ‘Gateral  gastrulation.” 

In  Stage  B  (Fig.  2)  when  the  caudal  amnio-serosal  fold  has 
covered  the  ligulate  portion  of  the  germ-band,  the  blasto¬ 
pore  presents  a  widening  of  its  anterior  end  at  a  point  which 
probably  lies  just  in  front  of  the  definitive  mouth.  This  widen¬ 
ing  was  observed  in  several  embryos,  and  I  therefore  take  it 
to  be  a  normal  occurrence.  It  also  has  its  homologue  in  the 
Doryphora  embryo  (see  my  Fig.  70,  PI.  XVIII,  ’89).  In  the 
stage  under  consideration  (Fig.  2)  the  anal  bifurcation  has 
grown  more  indistinct  and  is  apparently  about  to  disappear. 

The  closure  of  the  blastopore  proceeds  simultaneously  in 
two  directions :  from  its  anterior  end  backwards,  and  from  its 
posterior  end  forwards,  so  that  the  last  portion  of  the  groove 
to  disappear  lies  in  that  part  of  the  germ-band  which  is  to 
become  the  thoracic  or  baso-abdominal  region. 

In  sections  the  groove  is  seen  to  be  much  shallower  than  it 
appears  in  surface  view.  Along  its  whole  extent  its  floor  is 
somewhat  thickened  and  in  this  portion  —  destined  to  form  the 
mesentoderm  —  the  cells  soon  lose  their  columnar  shapes  and 
become  more  polygonal  in  outline  and  more  irregular  in  their 
arrangement.  The  groove  closes  in  such  a  way  that  no 
tubular  cavity  results  as  in  the  Coleoptera  and  Diptera;  the 
cells  at  the  edges  of  the  depression  appear  to  glide  over  the 
median  elements,  so  that  after  the  fusion  of  the  edges  in  the 
median  line  the  mesentoderm  has  the  form  of  a  solid  cord 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


39 


applied  to  the  inner  surface  of  the  germ-band.  The  process 
whereby  the  inner  layers  are  formed  is,  therefore,  a  slurred 
invagination.  In  this  respect  Xiphidium  resembles  Blatta. 

The  further  differentiation  of  the  mesentoderm  is  quite  as 
difficult  to  follow  in  Xiphidium  as  in  other  Orthoptera.  In 
these  stages  the  embryo  cannot  be  satisfactorily  isolated  from 
the  yolk  and  sectioned  by  itself,  and  so  friable  is  the  yolk  that  it 
is  almost  impossible  to  obtain  thin  sections  through  the  entire 
egg  by  the  ordinary  methods.  After  studying  a  few  series  of 
sections  obtained  by  means  of  the  celloidin  method  I  can,  how¬ 
ever,  affirm  that  the  invaginated  cells  give  rise  to  both 
entoderm  and  mesoderm.  The  former  has  a  bipolar  origin,  as 
has  been  made  out  in  the  higher  forms;  in  Apis  by  Grassi  (’84); 
in  Hydrophilus  by  Heider  (’89);  in  Dojyphora  by  myself  (’89); 
in  Musca  by  Voeltzkow  (’89)  and  by  Graber  (’89);  and  in 
Chalicodovia  by  Carriere  (’90).  The  anal  is  considerably  larger 
than  the  oral  formative  centre  and  its  elements  seem  to  arise 
in  part  from  the  bifurcation  and  in  part  from  the  deeper 
portion  of  the  blastopore  just  in  front  of  the  bifurcation. 

In  Xiphidium,  just  as  in  the  higher  Metabola,  a  pair  of 
entoderm-bands  grows  towards  the  baso-abdominal  region  from 
either  entoderm-pole.  Each  band,  consisting  of  only  one 
layer  of  much-flattened  cells,  meets  that  of  its  respective  side 
and  then  begins  to  envelop  the  yolk  by  proliferation  at  its 
ventral  and  dorsal  edges.  Transverse  sections  show  that 
at  first  the  bands  are  only  two  or  three  cells  in  breadth  and  that 
these  are  closely  applied  to  the  dorsal  faces  of  the  mesomeres 
which  are  formed  by  this  time. 

I  have  made  no  observations  on  the  relations  of  the  procto- 
daeum  to  the  posterior  end  of  the  blastopore,  but  in  regard  to 
the  anterior  end  and  its  relation  to  the  stomodaeum  my  results 
are  more  definite.  Figs.  32-34  represent  three  successive 
sections  through  the  head  of  an  embryo  in  Stage  D.  The  last 
section  (Fig.  34)  passes  through  the  stomodaeum  {sti)  which  is 
just  forming  as  a  rounded  depression  in  the  cephalic  ectoderm. 
Its  large  columnar  cells  are  regularly  arranged  and  have  their 
nuclei  in  the  inner  ends.  The  next  section  (Fig.  33)  passes 
just  in  front  of  the  stomodaeiim  and  cuts  two  masses  of  cells  in 


40 


WHEELER. 


[VOL.  VIII. 


the  median  line.  The  upper  of  these  masses  is  a  thickening 
of  the  ectoderm  distinctly  separated  on  either  side  from  the 
elements  of  the  same  layer  by  the  peculiar  character  of  its 
cells.  These  are  much  smaller  than  those  of  the  remaining 
ectoderm  and  stain  more  deeply,  especially  in  the  inner  portions 
of  the  layer.  The  lower  mass  of  cells  is  entirely  cut  off  from 
the  ectodermal  thickening,  though  its  elements  are  very  similar 
in  size  and  staining  qualities.  The  ectodermal  thickening 
marks  the  point  where  the  paired  labrum  is  about  to  appear 
{cf.  Fig.  35).  In  the  next  section  (Fig.  32),  which  also 
passes  through  the  labral  region,  we  again  meet  with  the 
thickening  of  the  ectoderm.  Unlike  its  portion  in  the  pre¬ 
ceding  section,  it  is  not  bounded  below  by  a  curved  line, 
but  juts  in  as  a  ragged  mass  of  cells,  in  which  it  is  possible  to 
distinguish  a  pair  of  lateral  wings  and  a  median  projection. 
The  median  portion  thus  proliferated  beyond  the  limits  of  the 
ectoderm,  is  the  anterior  or  oral  entoderm  centre  —  the  lateral 
wings  I  regard  as  mesodermal.  By  combining  Figs.  32  and 
33  the  flattened  mass  of  cells  underlying  the  ectoderm  in  the 
latter  section  is  seen  to  be  the  backward  continuation  of  the 
mesentoderm.  Section  Fig.  34  shows  that  this  median  unpaired 
mass  splits  into  two  masses,  one  on  either  side  of  the  mouth. 
In  this  paired  condition  the  bands  run  backwards  through  the 
trunk  of  the  embryo. 

Essentially  the  same  condition  of  the  germ-layers  in  front  of 
the  mouth  persists  till  the  labrum  is  definitely  formed,  as  I  have 
observed  in  a  few  series  of  sections.  It  is  difficult  to  account 
for  the  late  and  intimate  union  of  the  mesentoderm  with  the 
ectoderm  in  the  labral  region,  unless  we  suppose  that  the 
blastopore  originally  extended  as  far  forward  as  this  region  and 
here  closed  in  such  a  manner  that  the  three  layers  were  not  at 
once  separated  into  ectoderm  on  the  one  hand  and  mesentoderm 
on  the  other.  It  is  mainly  on  this  supposition  that  I  take  the 
labral  region  to  coincide  with  the  anterior  widening  of  the 
blastopore  seen  in  Fig.  2.  This  widening  probably  does  not 
coincide  with  the  stomodaeum,  but  lies  in  front  of  it,  and  the 
definitive  mouth  is  a  later  formation  arising  independently  from 
the  ectoderm  alone. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


41 


I  would  here  insert  a  few  observations  on  gastrulation  in 
Stag7iionia7itis  carolma,  Gryllus  hccticosics,  and  CEca7itJius 
7  live  Its. 

In  Fig.  12  the  egg  of  Stag77ioi7ia7itis  is  represented  in  out¬ 
line  for  the  purpose  of  showing  the  relatively  small  size  of  the 
germ-band  which  arises  as  in  other  forms  from  a  thickening  of 
the  blastoderm  on  the  ventral  face  of  the  yolk.  It  is  seen  to 
lie  somewhat  nearer  the  broad  posterior  than  the  pointed 
anterior  pole.  It  is  but  slightly  longer  than  broad,  and  its 
wider  anterior  end,  which  is  directed  towards  the  upper  pole 
of  the  egg,  foreshadows  the  procephalic  lobes.  Fig.  1 1  shows 
that  the  germ-band  of  the  Mantid,  unlike  that  of  Xiphidiimi, 
is  raised  above  the  niveau  of  the  yolk  and  has  its  marginal  cells 
sharply  separated  from  the  extra-embryonal  blastoderm  —  or 
serosa  —  as  it  is  now  called.  This  much  flattened  layer  is, 
nevertheless,  encroaching  on  the  edges  of  the  germ-band  to 
form  the  amnio-serosal  fold  (aiiis.).  At  the  anterior  edge  lies  a 
small  cluster  of  cells  (/.  0)  but  little  larger  than  those  of  the 
germ-band.  I  believe  that  these  may  represent  all  that  remains 
of  an  indusium  in  StagiTtoinantis . 

The  narrowly  pear-shaped  blastopore  is  very  short.  Sections 
show  it  to  be  a  deep  groove,  which  like  the  median  infolding 
of  other  forms  {po7ypJiora,  Hyd7'op]iihLS^  Musca,  etc.)  is  deep¬ 
est  posteriorly  and  grows  shallower  headward.  As  I  failed  to 
find  any  of  the  stages  immediately  following  the  one  figured,  I 
could  not  trace  out  the  formation  of  the  germ-layers. 

According  to  Bruce  (’86,  p.  17),  who  studied  the  same  species 
of  Stag77i077ta7itis,  “  When  the  union  of  the  folds  (of  the  amnion 
and  serosa)  is  effected  and  the  embryo  is  separated  from  the  sur¬ 
face  and  covered  ventrally  by  the  amnion,  the  under  layer  is 
formed,  as  in  Meloe  and  TJiyridopteiyx  as  an  ingrowth  from  the 
median  line  of  the  embryo.”  This  remark,  together  with  his 
Figs.  XLII-XLIV,  PI.  IV,  shows  that  he  could  not  have  ob¬ 
served  the  formation  of  the  layers  from  a  groove  and  that  he 
must  have  based  his  inference  on  a  stage  later  than  the  one  I 
have  figured. 

In  Gryllus  luctuosus  the  blastopore  is  more  like  that  of 
Xiphidiu77i.  The  outline  of  the  egg  is  shown  in  Fig.  14.  The 


42 


WHEELER. 


[VOL.  VIII. 


germ-band  is  relatively  much  larger  when  compared  with  the 
yolk-mass  than  the  germ-band  of  Stagmo^nantis .  It  arises  on 
the  ventral  surface  very  near  the  lower  pole.  That  such  is  the 
correct  position  of  the  embryo  may  be  easily  ascertained,  since 
the  mother-insect  thrusts  her  eggs  into  the  ground  with  their 
long  axes  perpendicular  to  the  surface.  In  a  glass  jar  con¬ 
taining  a  few  inches  of  earth,  many  eggs  were  deposited 
between  the  surface  of  the  glass  and  the  earth,  so  that  the 
exact  position  of  the  apical  pole  could  be  noted,  and  the  egg 
removed  and  hardened  with  this  pole  constantly  in  sight.  Thus 
it  was  possible  to  determine  the  exact  topographical  relations 
of  the  embryo  to  the  yolk  throughout  the  important  stages  of 
early  development. 

During  gastrulation  the  germ-band  of  Grylltcs  (Fig.  13)  is 
more  elliptical  and  somewhat  narrower  than  the  germ-band  of 
Stagmomantis .  Its  edges  are  also  distinctly  marked  off  from 
the  blastoderm  and  here,  too,  the  amnio-serosal  fold  (ams.) 
arises  along  the  entire  periphery.  The  blastopore  (d/.)  is  much 
narrower  than  the  corresponding  depression  in  Stagmomantis. 
It  is  deepest  posteriorly. 

The  discovery  of  an  invaginate  gastrula  in  Grylhis  made  it 
extremely  probable  that  this  stage  had  been  overlooked  in  the 
other  members  of  the  same  family  which  have  been  studied 
from  an  embryological  standpoint.  Neither  Korotneff  in  his 
study  of  Gryllotalpa  (’85),  nor  Ayers  in  his  study  of  CEcantJms 
(’84),  succeeded  in  finding  an  invagination.  I  was  unable  to 
secure  the  eggs  of  any  of  our  native  Gryllotalpce,  but  I  col¬ 
lected  a  great  number  of  CEcantIms  eggs  in  Ohio  during  the  last 
days  of  September.  An  examination  of  these  soon  convinced 
me  that  Ayers  had  not  seen  the  youngest  stages  in  the  develop¬ 
ment  of  the  germ-band.  The  youngest  germ-band  that  he 
figures  (Figs.  1-18)  lies  near  the  posterior  end  of  the  egg 
with  its  tail  pointing  towards  the  micropylar  pole.  According 
to  Ayers  “  A  tract  of  the  blastoderm  along  the  median  line  of 
the  ventral  (concave)  side,  lying  nearest  the  deep  or  primitively 
head-end  of  the  egg,  becomes  thickened  into  a  germinal  band, 
which  is  the  first  trace  of  the  body  of  the  embryo.”  But  this  is 
not  the  first  trace  of  the  body  of  the  embryo,  nor  does  it 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


arise  on  the  concave  face  of  the  egg.  The  germ-band  of 
CEcantJmSy  like  that  of  Grylltts.,  first  makes  its  appearance  as  a 
thickening  of  the  blastoderm  on  the  convex  surface  near  the 
lower  pole  of  the  egg.  This  convex  surface  is,  therefore,  the 
ventral  surface  and  the  micropyle  marks  the  “  primitively  head- 
end  ”  of  the  egg  as  is  shown  by  the  fact  that  the  procephaleum 
is  originally  directed  towards  this  and  not  towards  the  opposite 
pole,  which  Ayers  incorrectly  calls  the  “primitively  head-end.” 
The  germ-band,  however,  soon  leaves  its  position  on  the  convex 
ventral  surface  and,  moving  around  the  lower  pole  tail  first, 
comes  to  lie  on  the  concave  dorsal  yolk.  It  is  clear  that  Ayers 
could  not  have  seen  the  stages  preceding  the  arrival  of  the 
germ-band  on  the  dorsal  surface,  and  it  is  during  these  very 
stages  that  the  blastopore  forms  and  closes. 

Before  turning  the  lower  pole  the  germ-band  of  CEcantJms 
resembles  that  of  Stagmomantis .  Its  anterior  is  distinctly 
wider  than  its  posterior  end  and  represents  the  future  pro- 
cephalic  region.  A  narrow,  but  distinct  groove  runs  from  the 
oral  to  the  anal  end  as  in  the  forms  we  have  been  considering. 
At  the  posterior  end  the  groove  bifurcates  much  as  in 
Xiphidiiim.  That  this  median  groove  gives  rise  to  the  mesen- 
toderm  admits  of  little  doubt  after  what  has  been  said  of  other 
Orthoptera.  The  amnio-serosal  fold  appears  to  arise  simul¬ 
taneously  along  the  entire  margin  of  the  germ-band  as  in 
Gryllus. 

It  follows  from  the  observations  here  recorded,  fragmentary 
as  they  are  in  many  respects,  together  with  Graber’s  observa¬ 
tions  on  Stenobothrus,  that  the  Orthoptera  can  no  longer  be 
regarded  as  hors  de  ligne  so  far  as  the  formation  of  their 
germ-layers  is  concerned.  In  all  the  families  of  the  order, 
save  the  Phasmidae,  an  invaginate  gastrula  has  been  found, 
and  there  can  be  little  doubt  that  the  investigator  who 
is  so  fortunate  as  to  study  embryos  of  this  family  will  find  in 
them  essentially  the  same  process  of  germ-layer  formation. 


44 


WHEELER. 


[VOL.  VIII. 


The  view  is  now  pretty  generally  held  that  in  the  Insecta 
both  mesoderm  and  entoderm  arise  from  a  median  longitudinal 
furrow  —  the  former  layer  throughout  nearly  the  entire  length, 
the  latter  only  in  the  oral  and  anal  regions  of  the  germ-band  — 
and  that  the  vitellophags,  or  cells  left  in  the  yolk  at  a  time 
when  the  remaining  cleavage  products  are  traveling  to  the  sur¬ 
face  to  form  the  blastoderm,  take  no  part  whatsoever  in  the 
formation  of  the  mesenteron,  but  degenerate  in  sitn  and 
finally  undergo  dissolution.  Discussions  of  the  literature  on 
this  subject  are  to  be  found  in  the  papers  of  H eider  (’89)  and 
Graber  (’89,  ’90),  and  so  few  are  the  facts  accumulated  since 
these  resumes  were  penned  that  I  may  dispense  with  an 
historical  consideration  of  the  insect  germ-layers  in  the  present 
paper. 

In  the  interpretation  of  the  insect  gastrula  the  entoderm  has 
always  played  an  important  role.  The  origin  of  the  mesoderm 
has  long  been  known  and  has  been  duly  provided  for  in  the 
various  germ-layer  hypotheses  which  have  from  time  to  time 
been  advanced.  But  the  true  origin  of  the  lining  of  the  mid-gut 
has  been  ascertained  only  within  the  last  few  years,  so  that  we 
cannot  expect  to  find  an  adequate  treatment  of  this  layer  in 
the  older  theories.  Led  astray  by  what  had  been  observed  in 
Crustacea  and  Arachnida,  some  writers  chose  to  regard  the 
vitellophags  as  forming  the  mesenteron  and  shaped  their 
theories  accordingly  (Oscar  and  Richard  Hertwig,  ’8i).  But 
now  that  it  has  been  shown  that  the  vitellophags  take  no 
part  in  forming  the  lining  of  the  mid-gut,  their  morphological 
position  is  rendered  even  more  obscure,  and  we  are  brought 
face  to  face  with  the  question :  Are  the  vitellophags  a  portion 
of  the  entoderm  which  has  been  set  apart  very  early  in  develop¬ 
ment  for  the  purpose  of  yolk-liquefaction  or  are  they  an  entirely 
new  segregation  of  cells  belonging  to  none  of  the  conventional 
germ-layers  ?  Those  who  defend  the  former  alternative  main¬ 
tain  that  the  vitellophags  of  insects  are  entodermal  in  function 
inasmuch  as  they  digest  yolk  and  closely  resemble  the  amoeboid 
Crustacean  yolk-cells  which  are  actually  converted  into  the  lin¬ 
ing  of  the  mesenteron.  On  the  other  hand  it  is  urged,  that  as 
the  yolk-cells  arise  and  function  before  the  blastoderm  is  com- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


pleted  and  hence  some  time  before  the  germ-layers  are  formed, 
they  cannot  properly  be  assigned  to  the  entoderm.^ 

It  is  probably  best  to  await  the  results  of  further  investiga¬ 
tion  before  deciding  on  the  phylogenetic  relations  of  the  vitel- 
lophags  to  the  entoderm.  Heider  (’89)  has  also  expressed 
himself  to  this  effect  and  I  fully  endorse  his  opinion  when  he 
says:  ‘‘Immerhin  wird  man  vorlaufig  liber  vage  Vermuthungen 
nach  dieser  Richtung  nicht  hinauskommen,  und  ist  die  Frage 
nach  der  Auffassung  der  Dotterzellen  bei  dem  Nachweise,  dass 
sie  an  dem  Aufbau  des  Embryos  keinen  Antheil  nehmen, 
meiner  Ansicht  nach  von  geringerer  Wichtigkeit.” 

1  Besides  these  vitellophags  which  with  Cholodkowsky  (’91^)  we  may  call  the 
primary  yolk-cells  —  there  are  other  cells  which  detach  themselves  from  the  blas¬ 
toderm  or  embryo  and  enter  the  yolk.  These  Cholodkowsky  calls  secondary 
yolk-cells.  While  the  origin  of  the  primary  yolk-cells  has  been  quite  satisfactorily 
demonstrated,  this  cannot  be  said  of  those  of  the  second  class.  They  appear  to 
descend  into  the  yolk  at  different  times  in  different  species.  Thus,  according  to 
Patten  (’84),  all  the  cleavage  products  in  Neophylax  ascend  to  the  surface,  the 
yolk-cells  subsequently  descending  from  the  blastoderm.  I  claimed  a  similar  total 
migration  of  the  cleavage  products  to  the  surface  in  Blatta  (’89) ;  Cholodkowsky, 
however,  claims  that  some  of  the  cells  never  reach  the  surface,  but  remain  in  the 
yolk.  Be  this  as  it  may,  in  later  stages  I  believe  it  can  be  shown  that  cells  do 
migrate  into  the  yolk  from  the  embryo  and  especially  from  the  entoderm-centres. 
This  was  shown  by  me  to  be  the  case  in  Doryphora,  where  many  cells  pass  into  the 
yolk  from  either  entoderm  pole  (PI.  XIX,  Fig.  82;  PI.  XX,  Fig.  88).  I  have  since 
observed  an  exactly  similar  phenomenon  in  Telea  polyphemus  in  a  correspond¬ 
ing  stage  of  development.  Graber,  (’89,  p.  ii)  too,  has  made  a  similar  observa¬ 
tion  on  Melolo7itha,  w^here  he  saw  “  vom  invaginirten  Blastoderm wulst  aus  unter 
lebhaften  Theilungserscheinungen  ganze  Strbme  von  Zellen  in  den  Dotter  hinein- 
wandern,  Zellen  die  freilich  von  den  primaren,  gleichzeitig  vorkommenden  und 
auffallend  grosskernigen  Centroblastelementen  ganz  enorm  verschieden  sind,  und 
die  sich  iiberhaupt  durch  ihre  ganze  Beschaffenheit  als  unzweideutige  Abkommlinge, 
man  konnte  sagen  Auswiirflinge  eines  wahren  Keimblattes,  erweisen.”  So  far  as  the 
migrant  cells  described  in  Doryphora  are  concerned,  I  am  sure  they  come  from 
the  entoderm.  They  occur  only  at  or  near  the  entodermal  Anlagen  and  may  be 
traced  from  this  germ-layer  into  the  yolk.  These  cells  are  not  actively  dividing 
like  those  described  by  Graber,  but  actively  disintegrating.  (May  not  Graber 
have  mistaken  disintegration-figures  for  caryokinetic  figures  ?)  In  somewhat 
later  stages  no  traces  of  these  migrant  cells  are  to  be  found.  The  yolk  is 
segmented  at  the  time  of  their  leaving  the  entoderm  and  their  invasion  appears 
not  to  disturb  in  the  least  the  activities  of  the  vitellophags.  Whether  there 
is  any  relation  between  these  evanescent  entoderm  cells  and  the  “secondary 
mesoderm”  of  Reichenbach  (’86),  the  “spores”  of  F.  H.  Herrick  (’86),  or  the 
“chromatin  nebulee”  of  Bumpus  (’91)  is  a  question  which  cannot  be  answered  at 
present. 


46 


WHEELER. 


[VOL.  VIII. 


Among  those  who  take  a  decided  stand  on  the  relations  of 
the  vitellophags  to  the  definitive  entoderm,  Graber  and  Cholod- 
kowsky  may  be  mentioned.  Graber  (’89,  p.  lo),  after  intro¬ 
ducing  the  superfluous  and  inapplicable  term  “  centroblast,”  ^ 
says :  Dabei  nehme  ich  zugleich,  was  indessen  kaum  misbilligt 
werden  diirfte,  stillschweigend  auch  an,  dass  dieses  gegenwartig, 
wie  es  scheint,  von  der  Darm-  and  Gewebsbildung  ausgeschlos- 
sene  Zellenlager  auch  friiher  niemals  eine  dem  echten  Ento¬ 
derm  anderer  Thiere  entsprechende  Rolle  inne  gehabt  habe, 
sondern  vielmehr  dem  letzteren  gegeniiber  ein  neues,  wahr- 
scheinlich  mit  der  starkeren  Entwicklung  des  Dotters  im 
Zusammenhang  stehendes  Differenzirungsproduct  ist.” 

Cholodkowsky  (’9i®)  does  not  dismiss  the  matter  so  briefly. 
Like  Graber  he  draws  a  hard  and  fast  line  between  primary 
and  secondary  yolk-cells,  and  admits  no  phylogenetic  continuity 
between  the  vitellophags  and  the  definitive  entoderm.  The 
vitellophags  belong  to  none  of  the  germ-layers.  His  reasons 
for  not  regarding  them  as  a  precociously  segregated  portion  of 
the  entoderm  are  neither  new  nor  conclusive.  Like  other 
recent  investigators  he  admits  that  the  vitellophags  are  in  part 
digested  or  discharged  from  the  alimentary  tract  along  with 
the  remains  of  the  yolk  after  hatching.  But  he  is  not  satisfied 
that  the  yolk-cells  should  play  a  humble  role  in  the  insect 
economy.  Some  of  them  were  predestined  to  a  higher  function 
than  yolk-liquefaction  —  viz:  to  give  rise  to  the  blood,  the  fat- 
body  and  even  to  the  germ-cells.  He  therefore  supposes  that  the 
vitellophags  are  undifferentiated  cells.  But  this  supposition  is 
not  supported  by  the  facts.  That  they  are  on  the  contrary, 
considerably  specialized  is  shown  by  their  limited  function  and 
mobility,  their  gradual  and  prolonged  growth  (especially  in 
some  Orthoptera),  their  inability  to  undergo  caryokinesis  or 
even  akinesis,  and  their  suspicious  relations  to  the  bacteria¬ 
like  corpuscles  of  Blochmann.  On  a  priori  grounds  we  should 
not  expect  to  derive  whole  sets  of  tissues  from  such  specialized 
elements. 

1  Superfluous  because  we  have  enough  names  for  these  cells  already,  inap¬ 
plicable  because  the  termination  “blast”  is  properly  applied  only  to  cells  or 
tissues  of  a  germinal  character  —  not  to  decrepit  elements  like  the  yolk-cells. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


But  a  more  weighty  objection  may  be  adduced.  It  has  been 
shown  by  Heider  (’89)  and  Heymons  (’90),  not  to  mention  many 
previous  investigators,  that  the  fat-body  and  sexual-cells  arise 
from  the  mesoderm,  and  my  own  studies  fully  confirm  this  view. 
Concerning  the  origin  of  the  blood  there  is  some  doubt,  but  it 
should  be  stated  that  Cholodkowsky  has  made  no  satisfactory 
observations  of  his  own  on  this  point  and  that,  although  some 
facts  point  to  a  derivation  of  the  blood  from  the  yolk-cells, 
others  as  definitely  point  to  its  origin  in  the  mesodermal  layer. 

After  taking  for  granted  that  the  vitellophags  are  undiffer¬ 
entiated  cells,  that  they  have  nothing  and,  what  is  more,  never 
have  had  anything  to  do  with  the  entoderm,  and  that  they  give 
rise  to  blood-corpuscles,  adipose-tissue  and  germ-cells,  Cholod¬ 
kowsky  ushers  in  the  parablast  theory.  It  was  to  have  been 
hoped  that  this  theory  might  have  been  permitted  to  end  its 
days  in  peace  within  the  confines  of  vertebrate  embryology 
where  it  originated.  Fortunately,  however,  it  has  grown  too 
old  and  decrepit,  even  under  the  skillful  medical  treatment 
which  it  has  received  from  time  to  time,  to  be  of  any  service 
in  invertebrate  morphology. 

There  is  something  almost  ludicrous  in  Cholodkowsky’ s  ap¬ 
plication  of  the  parablast  theory  to  the  Insecta  when  we  con¬ 
sider  the  methods  which  he  employed  in  preparing  the  yolk  of 
the  Blatta  egg.  The  capsules  opened  at  both  ends  were  sub¬ 
jected  to  the  action  of  undiluted  Perenyi’s  fluid  for  12  hours 
and  the  eggs  after  treatment  with  the  customary  grades  of 
alcohol,  cleared  in  green  cedar  oil  24  hours.  Thence  they 
were  transferred  to  paraffine  (55—60°  C.)  and  left  3-5  hours. 
The  result  of  this  heroic  method  is  apparent  enough  in  the  dis¬ 
tortion  of  the  tissues,  but  its  effect  on  the  yolk  is  quite 
remarkable. 

Both  Blochmann  (’87)  and  myself  (’89)  described  the  yolk  of 
the  Blatta  egg  as  consisting  of  a  mass  of  homogeneous  and 
granular  albuminoid  bodies  sharply  polygonal  from  mutual 
pressure  and  interspersed  with  spherical  oil-globules.  We  also 
described  a  peculiar  distribution  of  the  polygonal  bodies;  those 
of  a  homogeneous  nature  constituting  an  oval  central  core  in¬ 
vested  with  the  granular  bodies.  I  further  claimed  that  the 


48 


WHEELER. 


[VOL.  VIII. 


Blatta  egg  exhibited  a  yolk-segmentation  which  though  faint 
and  appearing  late  was,  nevertheless,  comparable  to  the  yolk- 
segmentation  in  such  forms  as  Doryphora. 

Cholodkowsky  (’91®)  thus  describes  the  yolk:  ‘‘  So  kann  ich,  z. 
B.,  nicht  bestatigen,  dass  der  Dotter  aus  einzelnen  polygonalen 
Dotterkorpern  bestehe,  wie  derselbe  von  Blochmann  (und 
Wheeler)  beschrieben  und  abgebildet  wird.  Der  ganze  Dotter 
besteht  aus  einer  continuirlichen  plasmatischen  Substanz,  deren 
Vacuolen  grossere  und  kleinere  Fetttropfen  enthalten.  Die 
Continuirlichkeit  der  Dottermasse  tritt  nun  um  so  deutlicher 
hervor,  je  besser  die  Objekte  conservirt  sind.  Das  Bild  (ich 
mochte  sagen,  das  Trugbild)  der  polygonalen  Dotterkorper  ent- 
steht  durch  Bersten  des  Dotters  nach  der  Bearbeitung  mit 
nicht  ganz  passenden  Reactiven.”  ....  ^‘Auch  kann  ich 
die  Blochmann’sche  Unterscheidung  des  ‘inneren’  und 
^ausseren’  Dotters  nicht  annehmen;  der  ganze  Unterschied 
in  den  Farbungsverhaltnissen  der  beiden  angeblichen  Theile 
des  Dotters  lasst  sich  einfach  dadurch  erklaren,  dass  die  Farbe 
aus  den  peripherischen  Theilen  des  Eies  leichter  als  aus  den 
inneren  mit  Saure  ausgezogen  wird.”  And  at  p.  58  he  remarks: 

“  Es  ist  bemerkenswerth,  dass  bei  Blatta  germanica  eine 
Dotterzerkliiftung  vollkommen  fehlt.  Ich  kann  also  mit 
Wheeler  nicht  iibereinstimmen,  wenn  er  sagt  (p.  359),  dass  bei 
Blatta  der  Dotter,  wenn  auch  sehr  spat  (nach  Bildung  der 
Extremitaten)  sich  furchen  soil;  hochst  wahrscheinlich  war 
Wheeler  zu  dieser  irrigen  Annahme  durch  die  ausserordentliche 
Briichigkeit  des  Dotters  verleitet.” 

On  reading  these  criticisms  I  re-examined  my  preparations 
and  must  emphatically  re-assert  what  I  claimed  in  my  descrip¬ 
tion  of  the  yolk  of  the  Blatta  egg.  Among  my  preparations  I 
find  several  mature  ovarian  eggs  hardened  in  Perenyi’s  fluid  — 
not,  however,  treated  with  that  vigorous  reagent  for  12  con¬ 
secutive  hours  —  and  these  show  the  yolk-bodies  very  distinctly 
as  polygonal  masses.  There  are  no  traces  of  a  ‘‘Bersten  des.i 
Dotters.”  Eggs  killed  in  ordinary  alcohol  and  mounted  in  toto\^ 
show  the  polygonal  yolk-bodies  distinctly  and  in  these  same 
specimens  the  distribution  of  the  different  yolk-elements  may 
be  followed  by  carefully  focusing.  That  Cholodkowsky  should 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  49 

be  unable  to  detect  the  outlines  of  the  segments  in  the  yolk  of 
eggs  treated  for  half  a  day  with  Perenyi’s  fluid  is  not  surpris¬ 
ing,  especially  as  this  segmentation  is  of  very  short  duration  in 
Blatta  as  in  other  Orthoptera.  It  is  present,  however,  as  I 
have  convinced  myself  from  eggs  mounted  in  toto  and  from 
sections. 

If  prolonged  immersion  in  Perenyi’s  fluid  can  bring  about  a 
complete  fusion  of  the  yolk-bodies  and  an  obliteration  of  their 
true  structure,  what  must  be  its  effect  on  the  vitellophags 
scattered  through  the  yolk  ?  And  how  much  importance  are 
we  to  attach  to  Cholodkowsky’s  assertion  that  the  fat-body, 
blood-corpuscles  and  sexual-cells  arise  from  the  vitellophags, 
and  to  the  parablast  theory  as  applied  to  the  Blatta-owwm. } 

Let  us  return  from  this  digression  to  the  germ-layers.  The 
curious  fact  that  the  definitive  entoderm  of  the  Insecta  arises 
from  two  separate  centres  —  one  oral  and  the  other  anal  —  is 
too  recent  to  have  given  rise  to  much  speculation.  Since 
the  entoderm  of  other  animals  arises  from  a  single  centre  it  is 
tacitly  assumed  that  such  must  originally  have  been  the  case 
with  the  Insecta,  and  that  the  present  bipolar  condition  must  be 
due  to  secondary  modification.  Starting  with  this  postulate, 
there  are,  of  course,  many  ways  in  which  the  bipartition  of  the 
original  unipolar  entoderm  may  be  supposed  to  have  taken 
place.  Two  of  these  possibilities  are  worked  out  in  the  hypo¬ 
theses  of  Kowalevsky  (’86)  and  Cholodkowsky  (’9i^). 

Kowalevsky  has  expressed  his  views  so  clearly  and  con¬ 
cisely  that  I  cannot  do  better  than  quote  his  own  words: 
“Wenn  wir  jetzt  versuchen,  diese  Bildung  des  Ento-  und 
Mesoderms  bei  den  Musciden  mit  der  Bildung  dieser  Blatter 
bei  anderen  Thieren  zu  vergleichen,  so  sehen  wir  erstens,  dass 
hier  auch  eine  Art  sehr  in  die  Lange  ausgezogener  Gastrula 
entsteht,  und  dass  aus  dem  eingestiilpten  Teil  das  Ento-  und 
Mesoderm  sich  bildet.  Also  in  diesen  allgemeinen  Ziigen 
finden  wir  eine  Uebereinstimmung.  Es  scheint  mir  aber,  dass 
die  Parallele  noch  weiter  gezogen  werden  kann.  Namentlich 
wenn  wir  der  Bildung  des  Ento-Mesoderms  bei  Sagitta  uns 
erinnern,  so  finden  wir  bei  derselben  dass  der  eingestiilpte 
Teil  des  Blastoderms  in  drei  parallele  Sacke  zerfallt,  von 


50 


WHEELER. 


[VOL.  VIII. 


denen  der  mittlere  das  Entoderm  liefert,  die  seitlichen  aber  das 
Mesoderm.  Bei  den  Musciden  entsteht  auch  eine  solche  Ein- 
stiilpung  wie  bei  Sagitta,  und  auch  der  mittlere  Teil  —  aller- 
dings  nur  an  beiden  Enden  vorhanden  —  liefert  das  Entoderm, 
die  seitlichen  Teile  liefern  das  Mesoderm:  also  ahnlich  dem, 
was  wir  bei  der  Sagitta  beobachten.  Um  die  Aehnlichkeit 
weiter  zu  fiihren,  kann  vorausgesetzt  werden,  dass  bei  der  so 
in  die  Lange  gezogenen  Gastrula  der  Insekten  der  mittlere,  das 
Entoderm  liefernde  Sack  so  ausgezogen  ist,  dass  er  in  der 
Mitte  ganz  verschwindet  und  nur  an  seinem  vorderen  und 
hinteren  Ende  bestehen  bleibt.  Bei  dieser  Auffassung  wird  es 
von  selbst  schon  folgen,  dass  die  sich  schliessende  Rinne  fast 
auf  ihrer  ganzen  Lange  nur  das  Mesoderm  liefert. 

Jetzt  bleibt  noch  die  Frage  iibrig:  wie  verhalten  sich  die 
Flachen  der  Gastrula  zu  den  Flachen  des  sich  bildenden  Ento¬ 
derms.  Bei  der  Sagitta  wird  die  aussere  Oberflache  der  Blastula 
nach  der  Einstiilpung  zur  inneren  Oberflache  des  Darmkanals, 
d.  h.  die  Seiten  der  Zellen,  welche  bei  der  Blastula  nach  aussen 
gerichtet  waren,  werden  im  Darmkanal  nach  seinem  Lumen 
gerichtet.  Bei  den  Insekten  kann  dasselbe  auch  vorausgesetzt 
werden.  Wenn  wir  uns  die  eingestulpte  Rinne  vorstellen,  so 
sind  deren  Oberflachen  ganz  ahnlich  gelagert  wie  bei  der 
Gastrula;  wenn  wir  weiter  die  Bildung  der  beiden  Entoderm- 
anlagen  dem  mittleren  Sacke  der  Sagitta  vergleichen,  so  bleibt 
die  Lagerung  der  Zellenfiachen  noch  ganz  dieselbe.  Wenn  wir 
dann  voraussetzen,  dass  der  mittlere  Sack  durch  die  weite  Aus- 
breitung  und  durch  das  Eindringen  der  Masse  des  Dotters 
gewissermassen  in  seinen  vordern  und  hintern  Teil  zersprengt 
ist,  so  kommt  der  Dotter  ins  innere  des  hypothetischen  Sackes, 
und  die  Zellen,  die  den  Dotter  bedecken,  werden  zu  dem 
Dotter  in  derselben  Beziehung  stehen,  wie  bei  der  Sagitta  zu 
der  eingestiilpten  Flache.” 

A  few  years  after  these  remarks  were  written  H eider  (’89) 
and  myself  (’89)  at  about  the  same  time  published  observations 
on  the  Coleopteran  germ-layers  which  seemed  to  support  the 
hypothesis  of  the  celebrated  Russian  embryologist.  As  further 
support  to  Kowalevsky’s  view  I  believe  we  may  point  to  such 
gastrulas  as  that  of  Stagmomantis  described  above.  In  this 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


gastrula,  which  is  so  very  short  and  broad,  we  may  suppose 
that  the  oral  and  anal  entoderm-centres  are  really  continuous, 
covering  the  floor  of  the  blastopore  from  end  to  end.  In  sec¬ 
tions,  it  is  true,  I  failed  to  detect  any  differentiation  of  the 
cells  forming  the  walls  of  the  furrow,  into  entodermal  and 
mesodermal  elements,  but  this  would  also  be  the  case  in  the 
elongated  gastrulas  of  other  insects  in  a  correspondingly  early 
stage  {XipJndium^  Doryphora).  As  favoring  a  purely  mechan¬ 
ical  separation  of  an  originally  single  entoderm  Anlage,  it  may 
be  noted  that  the  most  rapid  elongation  of  the  germ-band 
occurs  at  a  time  when  the  entoderm  is  differentiating  from  the 
mesentoderm  Anlage.  There  is  probably  more  than  an  acci¬ 
dental  correlation  of  these  two  processes.  During  this  time 
some  germ-bands  {Doryphora,  Xiphidiufr)  double  their  length. ^ 
Inasmuch  as  the  lengthening  of  the  superficial  layers  of  the 
embryo  is  much  more  rapid  than  the  differentiation  of  the 
entoderm,  this  germ-layer  must  lag  behind.  In  most  insects 
the  embryo  is  at  the  time  of  its  greatest  elongation  much  longer 
than  the  yolk-mass  and  must  again  shorten  to  the  length  of  this 
mass,  so  that  a  rapid  proliferation  of  the  entoderm  may  be 
superfluous,  since  this  layer  would  have  to  readjust  itself  to 
the  yolk  with  the  contraction  of  the  embryo.  It  may,  therefore, 
be  an  advantage  for  the  entoderm  to  be  somewhat  retarded  in 
its  growth.  In  the  Orthoptera,  where  the  embryo  lengthens 
rapidly,  shortens,  and  then  lengthens  again  to  envelope  the  yolk 
we  may  suppose,  for  reasons  to  be  given  in  the  sequel,  that 
yolk  has  been  acquired.  This  seems  also  to  be  suggested  by 
the  histological  structure  of  the  embryonic  entoderm;  this 
layer  consisting  of  large  polygonal  cells  in  the  Coleoptera, 
which  have  only  a  medium  amount  of  yolk,  while  in  the 
Orthoptera  the  attenuate  entoderm-bands  consist  of  a  very 
few  flat  cells. ^ 

It  is  probable  that  when  more  forms  have  been  carefully 

1  Blatta  forms  a  very  rare  exception  in  this  respect. 

2  A  very  similar  condition  may  be  observed  in  the  case  of  the  blastoderm.  In 
the  Coleoptera  and  Diptera,  which  have  a  medium  or  small  amount  of  yolk,  the 
newly  formed  blastoderm  is  a  deeply  columnar  epithelium  ;  in  the  Orthoptera  it  is 
a  true  pavement  epithelium. 


52 


WHEELER, 


[VOL.  VIII. 


studied  a  method  of  entoderm  formation  midway  between  the 
unipolar  and  bipolar  methods  will  be  found  to  obtain  in  some 
insects.  We  must  admit  that  a  contribution  of  elements  to  the 
entoderm  from  the  interpolar  region  of  the  furrow  is  not  with 
certainty  precluded  in  several  of  the  species  which  have  been 
studied.  Thus  Heider,  (’89)  while  inclined  to  believe  that  such 
a  contribution  does  not  take  place  in  the  anterior  portion  of 
the  germ-band,  believes  that  it  may  take  place  in  the  posterior 
abdominal  segments.  Cholodkowsky  (’9i^)  is  inclined  to  accept 
a  still  more  diffuse  origin  for  the  entoderm.  “  Untersuchungen 
zahlreicher  Schnittserien  machen  es  wahrscheinlich,  dass  an 
verschiedenen  Stellen  des  Keimstreifens  sich  einzelne  Zellen 
vom  ausseren  Blatte  abspalten  und  an  der  Bildung  des  inneren 
Blattes  betheiligen,  so  dass  die  Entstehungsart  des  letzteren 
sehr  complicirt  erscheint.” 

Starting  with  the  same  postulate  as  Kowalevsky,  viz:  that 
the  bipolar  is  derivable  from  a  unipolar  condition  of  the  ento¬ 
derm —  Cholodkowsky  (’91®-  ’91^)  proceeds  to  account  for  this 
phenomenon  in  a  very  different  way  from  his  compatriot.  He 
takes  the  small,  round  blastopore  of  As  tarns,  stretches  it  till  it 
equals  the  insect  blastopore  in  length,  introduces  a  number  of 
modifications  —  such  as  the  median  groove  and  the  pairs  of 
lateral  depressions  and  believes  that  he  has  found  an  explana¬ 
tion  ‘‘  sehr  klar  und  ungezwungen  ”  for  all  the  different  blasto¬ 
pores,  not  only  in  the  Insecta,  but  also  in  the  meroblastic  eggs 
of  vertebrates.  It  was  Kleinenberg  who  said :  “  Gewagte  Hypo- 
thesen,  kiihne  Schliisse  nutzen  der  Wissenschaft  fast  immer, 
die  Schemata  schaden  ihr,  wenn  sie  die  vorhandenen  Kenntnisse 
in  eine  leere  und  dazu  noch  schiefe  Form  bringen  und  bean- 
spruchen  tiefere  Einsicht  zu  geben.”  The  latter  part  of  this 
aphorism  seems  to  be  particularly  applicable  to  Cholodkowsky’s 
exposition.  As  Graber  (’9i)  has  briefly  pointed  out,  there  are  no 
grounds  for  comparing  the  Astactis  blastopore  with  the  entire 
insect  blastopore.  In  the  Decapods  this  orifice  is  confined  to 
the  anal  region  and  if  comparable  at  all  to  the  median  furrow 
in  insects,  must  be  compared  with  the  caudal  entoderm  pole. 
This  is  all  that  is  admissible,  since  the  mesoderm  of  the 
Decapoda  arises  from  the  anterior  lip  of  the  blastopore  and 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


proliferates  headward.  That  such  is  its  origin  has  been  shown 
by  Bobretzky  and  Reichenbach  for  Astactts  (’86),  by  Paul 
Mayer  for  Ettpagurits  igii),  and  by  Bumpus  for  HomartLs  (’9i). 
To  Cholodkowsky  both  the  extent  and  position  of  the  blastopore 
are  of  little  consequence  as  is  abundantly  evident  from  his 
reply  to  Graber’s  well-founded  objection.  It  is  this  very  neglect 
of  what  are  generally  and,  I  believe,  rightly  considered  two  of 
the  most  important  matters  in  the  discussion  of  the  germ- 
layers,  which  stamps  Cholodkowsky’ s  hypothesis  as  superficial 
and  inadequate. 

There  is,  however,  one  redeeming  suggestion  in  his  hypo¬ 
thesis,  viz:  that  the  diverging  grooves  at  the  posterior  end  of 
the  blastopore  in  insects  may  correspond  to  the  “  Sichelrinne  ” 
of  vertebrates.  Certainly  the  relations  of  the  grooves  to  the 
median  furrow  \x\.  XipJiidium  (see  Fig.  i.)  closely  resemble  in 
surface  view  the  relations  of  the  “  Sichelrinne  ”  to  the  primitive 
streak  in  the  chick  as  figured  by  Roller  (’8i)  and  in  the  triton 
as  figured  by  Oscar  Hertwig  (’90,  p.  99). 

While  most  investigators  probably  agree  with  Kowalevsky 
and  Cholodkowsky  in  deriving  the  bipolar  from  a  unipolar  con¬ 
dition  of  the  entoderm,  Patten  does  not  share  this  view  (’90). 
In  his  opinion,  which  is  based  on  Kleinenberg’s  interpretation 
of  the  gastrula,  the  blastopore  is  restricted  to  the  oral  region, 
and  such  depressions  as  occur  at  the  posterior  end  of  the  germ- 
band,  as  well  as  the  formation  of  teloblasts  in  that  region,  are 
supposed  by  him  to  have  no  connection  with  the  blastopore, 
but  to  be  merely  the  instruments  of  unipolar  growth.  ‘‘The 
Arthropod  body  represents  an  outgrowth  from  the  trochosphere, 
but  the  trochosphere  itself,  the  coelenterate  stage,  has  disap¬ 
peared.  Hence  there  is  no  such  thing  as  a  gastrula  in 
Arthropods  and  strictly  speaking,  no  germ-layers.”  It  is  clear 
that  this  view  must  stand  or  fall  with  Kleinenberg’s  theoretical 
conclusions  on  which  it  is  based,  and  we  may  venture  to  say 
that  E.  B.  Wilson’s  recent  work  (’90)  has  rendered  this  founda¬ 
tion  very  insecure,  notwithstanding  Patten’s  rather  confidant 
assertion  that  Lopadorhyiicims  it  is  certain  that  the  greater 
part  of  the  mesoderm  arises  from  the  ectoderm  at  the  growing 
tip  of  the  tail,  and  has  nothing  to  do  with  primitive  mesoderm.” 


54 


WHEELER. 


[VOL.  VIII. 


But  it  would  be  out  of  place  to  consider  the  widest  bearings 
of  Patten’s  hypothesis  in  this  paper  since  I  am  concerned  with 
it  only  in  so  far  as  it  bears  on  the  germ-layers  of  insects. 
Starting  with  the  assumption  that  the  blastopore  is  confined 
to  the  mouth,  he  attempts  to  show  that  the  median  furrow  is  a 
purely  secondary  structure.  “That  the  median  furrow  of  in¬ 
sects  is  merely  an  ontogenetic  adaptation  is  sufficiently  evident 
from  the  fact  that  it  may  be  present  or  absent  in  closely 
related  forms.”  This,  however,  is  not  the  case.  On  the  contrary 
the  furrow  or  a  slight  modification  of  it  is,  we  have  every  rea¬ 
son  to  suppose,  universally  present  in  the  Insecta,  at  least  in 
the  Pterygota,  and  this  wide  occurrence  of  the  structure  is  one 
of  the  surest  indications  of  its  high  antiquity  and  phylogenetic 
importance. 

In  the  latter  part  of  his  discussion  Patten  admits  that  there 
are  “  structures  in  Arthropods  which  may  represent  remnants 
of  gastrulas.  For  example,  if  the  mouth  and  oesophagus  of 
Arthropods  is  primitive  —  and  there  is  no  reason  to  suppose  it 
is  secondarily  acquired  —  then  we  must  look  for  primitive 
entoderm  at  its  inner  end.  I  have  figured  in  ‘  Eyes  of 
Acilius,’  at  the  very  anterior  end  of  the  embryo,  a  great  sac 
of  entoderm  cells  which  probably  arise  by  invagination, 
although  the  process  was  not  directly  observed.  The  sac, 
which  soon  opens  outward  by  the  oesophagus,  afterwards 
becomes  solid,  and  finally  is  converted  into  two  longitudinal 
bands,  one  on  either  side,  extending  backwards  to  the  middle 
of  the  body,  where  they  become  continuous  with  similar  bands 
extending  forwards  from  the  posterior  end  of  the  embryo.” 
Patten  admits  that  true  entoderm  is  formed  at  two  widely 
separated  regions  of  the  body,  but  he  implies  that  only  the 
anterior  centre  is  comparable  to  the  entoderm  of  other  animals, 
the  posterior  centre  being  a  new  and  purely  adaptive  formation. 
It  is  just  here  that  his  theory  appears  to  me  to  fail,  since  it 
does  not  explain  why  the  oral  and  anal  centres  should  resemble 
each  other  so  very  closely  in  origin,  method  of  growth  and 
histological  structure. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  55 


III.  The  Indusium  and  its  Homologues. 

In  none  of  the  Pterygota  hitherto  studied  has  there  been 
found  any  trace  of  a  structure  comparable  to  the  indusium 
of  XipJiidiiim  and  OrcJielimum.  The  organ  appears  to  have 
been  retained  by  the  Locustidae  and  completely  lost  by  the 
embryos  of  other  winged  insects.  In  some  of  the  Apterygota, 
however,  there  is  an  embryonic  organ  which  gives  a  clue  to  the 
possible  homologues  of  the  indusium.  I  allude  to  the  so-called 
“micropyle”  of  the  Poduridae. 

During  the  summer  of  ’91  I  was  so  fortunate  as  to  secure  the 
eggs  of  Anurida  maritima  in  great  numbers.  They  are  much 
larger  than  any  of  the  Poduran  eggs  hitherto  studied  —  so  large 
that  they  may  be  removed  from  their  choria  by  means  of 
dissecting-needles  and  partially  stained  for  surface  views.  It 
is  also  an  easy  matter  to  obtain  good  sections.^  When  first 
deposited  the  eggs  are  provided  with  a  thin  transparent  chorion 
and  vitelline  membrane,  but  after  cleavage,  which  is  total,  is 
completed  and  the  blastoderm  formed,  a  yellow,  peculiarly 
striated  chitinous  membrane  is  secreted  from  the  surface.  The 
egg  then  enlarges  till  the  chorion  and  vitelline  membrane  are 
burst.  The  striated  membrane  was  described  by  Ryder  (’86), 
but  he  failed  to  observe  that  it  is  attached  to  a  large  circular 
ring  —  the  micropyle.”  In  section  (Fig.  V)  this  organ  is  seen 
to  be  a  very  decided  thickening  of  the  blastoderm  which  at 
this  time  covers  the  whole  yolk-mass  as  a  single  layer  of 
minute  columnar  cells.  In  the  “micropyle”  the  cells  and 
nuclei  are  much  enlarged  and  often  considerably  vacuolated. 
Surface  views  prepared  according  to  the  partial  staining  method 
show  that  the  embryo  is  already  faintly  outlined  on  the  yolk  and 
that  the  ring-shaped  organ  lies  just  in  front  of  its  head  (Fig.  VI). 
The  egg  being  spherical,  the  embryo  is  curled  in  a  semicircle 
and  the  ^‘micropyle”  thus  comes  to  lie  on  the  dorsal  surface 
nearer  the  head  than  the  tail  of  the  germ-band.  In  the  figure 
a  more  advanced  embryo  is  represented  as  spread  out  on  a  flat 

1  I  mention  this  because  the  few  fragmentary  accounts  that  have  been  published 
on  the  development  of  the  Poduridse  are  based  on  the  study  of  the  embryo  viewed 
through  the  chorion  and  other  envelopes.  This  has  given  rise  to  some  errors 
which  I  hope  to  point  out  in  a  future  paper. 


56 


WHEELER. 


[VOL.  VIII. 


surface.  The  resemblance  of  the  “micropyle”  to  the  indusium 
is  apparent  at  a  glance  {cf.  Fig.  2,  PI.  I).  I  have  followed  the 
organ  in  Amirida  through  the  later  stages  by  means  of  sections 
and  find  that  it  persists  for  some  time  as  a  simple  thickening 
of  the  blastoderm,  still  connected  with  the  peculiar  striated 
membrane  which  stands  away  from  the  surface  of  the  blasto¬ 
derm  at  all  other  points.  Finally,  when  the  embryo  has 
become  flexed  dorsoventrally  and  the  body-walls  are  closed, 
it  sinks  into  the  yolk  and  is  absorbed. 


Fig.  V. 


Median  section  of  the  egg  of  Anurida  niaritima.  d.o.,  “micropyle”;  bid., 

blastoderm. 

Although  much  simpler  in  its  structure,  I  do  not  hesitate 
to  homologize  this  “micropylar”  organ  in  Amirida  and  the 
Poduridae  in  general  with  the  indusium  of  Xiphidium.  A  pos¬ 
sible  objection  to  this  homology,  on  the  ground  that  the  indu¬ 
sium  arises  on  the  ventral  face  of  the  egg,  while  the  Podurid 
“  micropyle  ”  is  dorsal,  has  little  weight,  since  the  organ  bears 
in  either  case  the  same  relation  to  the  head  of  the  embryo. 
Provided,  therefore,  the  egg  of  Amirida  were  to  acquire  yolk 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


j?;  #% 

t  K.Ji: 


and  become  greatly  elongated,  like  the  Xiphidium  egg,  the 
micropylar  organ  must  come  to  lie  on  the  same  surface  of  the 
yolk  as  the  germ-band. 

It  has  been  repeatedly  suggested,  and,  I  believe,  on  very 
good  grounds,  that  the  Podurid  “micropyle”  is  the  homologue 

of  the  crustacean  “dorsal  organ.”  In 
both  groups  the  organ  arises  soon 
after  the  germ-band  is  mapped  out  on 
the  yolk,  and  in  both  groups  it  is  a 
circular  or  oval  thickening  of  the  blas¬ 
toderm  lying  in  the  median  dorsal  line 
distinctly  nearer  the  head  than  the 
telson.  In  the  Crustacea  its  centre 
often  shows  a  depression  to  the  walls 
of  which  the  Blastoderm-haut  is 
attached,  while  standing  away  from 
the  surface  of  the  egg  at  other  points. 
An  exactly  similar  condition  obtains 
in  the  Poduridae;  a  slight  depression 
marks  the  centre  of  the  organ  in 
Amirida,  while  in  Amirophorus  (Le- 
moine,  ’87)  there  appears  to  be  a  deep 
pit  at  the  attachment  of  the  chitinous 
envelope.  This  depression  is  com¬ 
parable  to  the  depression  seen  in 
Fig.  3  in  Xiphidmm,  where  the  cir¬ 
cular  fold  is  encroaching  on  the 
disk. 

Up  to  the  stage  represented  in 
the  figure  just  referred  to,  the  indu- 
sium  will  bear  close  comparison 
with  the  crustacean  “  dorsal  organ.” 
In  the  first  stages  of  its  spread¬ 
ing  it  also  resembles  to  some  extent 
the  saddle-shaped  “dorsal  organ” 
of  OitiscuSf  Porcelho,  and  Ligia. 


VI  ^  V . 

5.W - 


.....aYV/. 


Fig.  VI. 

Embryo  of  Anurida  mari- 
tima  spread  out  on  a  flat 
surface,  d.o.,  “  micropyle  ” /(5., 
labrum;  yc/.,procephalic  lobe; 
at,  antenna;  tc.  ap.,  minute  ap¬ 
pendage  of  the  tritocerebral 
segment;  mandible;  mx^, 
first  and  second  maxillae; 
P^-p^^  first  to  third  thoracic 
appendages;  first  abdom¬ 
inal  appendage  (=  left  half  of 
collophore);  ap"^,  second  ab¬ 
dominal  appendage;  an.,  anus. 


But  beyond  this  point  it  differs 
widely  from  its  homologues,  and  it  is  difficult  to  see  why  it 


WHEELER. 


[VOL.  VIII. 


5S 

should  persist,  and  instead  of  sinking  into  the  yolk,  envelop 
the  whole  egg,  secrete  a  granular  and  thereupon  a  chitinous 
layer,  and  finally,  during  revolution  take  on  the  function  of  a 
true  serosa.  That  the  organ  is  rudimental  is  shown  by  its 
tendency  to  vary,  especially  during  the  earlier  stages  of  its 
development;  that  it  still  performs  some  function  is  indicated 
by  its  somewhat  complicated  later  development  and  by  its  sur¬ 
vival  in  but  very  few  forms  out  of  the  vast  group  of  Ptery- 
gotous  insects.  This  seeming  paradox  may  be  explained,  if 
we  suppose  that  the  indusium  was  on  the  verge  of  disappear¬ 
ing,  being  the  last  rudiment  of  some  very  ancient  structure. 
As  such  a  rudiment  it  no  longer  fell  under  the  influence  of 
natural  selection,  and  for  this  reason  began  to  vary  consider¬ 
ably  like  other  rudimental  organs.  Some  of  these  fortuitous 
variations  may  have  come  to  be  advantageous  to  the  embryo, 
and  were  perhaps  again  seized  upon  by  natural  selection;  the 
nearly  extinct  organ  being  thus  resuscitated  and  again  forced 
to  take  an  active  part  in  the  processes  of  development. 

Pursuing  the  homologies  of  the  indusium  still  further  we 
come  to  the  Arachnida,  where  we  find  in  the  primitive  cumulus 
of  spiders  a  structure  comparable  in  many  ways  to  the 
Podurid  “micropyle,”  as  v.  Kennel  (’85, ’88)  and  Lemoine  (’87) 
have  suggested.  There  is,  however,  so  much  difference  of 
opinion  regarding  the  position  and  signification  of  the  primitive 
cumulus  that  I  should  hardly  be  willing  to  agree  with  these 
authors,  were  it  not  for  two  of  ClaparMe’s  figures  of  the 
Pholms  embryo  (’62,  Figs.  6  and  7,  PI.  I).  These  show  in  the 
median  dorsal  line  a  thickening  which  forcibly  recalls  the 
“micropyle”  of  Anurida.  Still  it  must  be  admitted  that 
Clapar^e  has  failed  to  prove  the  identity  of  this  thickening 
with  the  primitive  cumulus. 

In  Pentastomids  the  ‘Tacette  ”  or  “cervical  cross”  described 
by  Leuckart  (’60)  and  Stiles  (’9i)  is  very  probably  the  homo- 
logue  of  the  crustacean  dorsal  organ  and  the  insect  indusium. 

Although  no  homologous  structure  has  yet  been  detected  in 
the  Myriopoda,  the  occurrence  of  a  dorsal-organ-like  structure 
in  such  widely  separated  groups  as  the  Hexapoda,  Araneina, 
Pentastomidae,  and  Crustacea  is  sufficient  reason  for  regarding 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


59 


it  as  exceedingly  ancient  and  as  well-developed  before  the 
existing  subdivisions  of  the  Arthropoda  were  established.  To 
seek  a  homologue  of  the  ‘‘dorsal  organ”  among  existing 
annelids  may  be  regarded  by  some  as  a  hopeless  undertaking. 
Still  I  would  call  attention  to  Apathy’s  observation  (’88)  on 
Clepsiiie  biocidata.  The  adult  of  this  species  has  long  been 
known  to  possess  a  chitinous  plate  in  the  median  line  between 
the  head  and  the  praeclitellum.  Apathy  finds  that  this  plate  is 
the  remnant  of  an  embryonic  sucking-disk,  the  glandular  cells 
of  which  secrete  a  bundle  of  byssus-like  threads  that  harden 
on  contact  with  the  water  and  serve  to  anchor  the  undeveloped 
young  to  the  ventral  concavity  of  the  mother-leech.  A  similar 
organ  is  also  found  in  the  young  of  Clepsine  heteroclita.  It 
is  certainly  no  great  step  from  this  embryonic  sucking-disk  of 
the  Hirudinea  to  the  Phyllopod  “cervical  gland”  which  is  also 
used  as  a  sucker,  and  which  Fritz  Muller  (’64)  and  Grobben 
(’79)  regard  as  homologous  with  the  “dorsal  organ”  of  the 
.  Amphipoda. 

IV.  The  Envelopes  and  Revolution  of  the  Insect  Embryo. 

I.  The  Amnion  and  Serosa. 

The  formation  of  two  cellular  envelopes,  the  amnion  and  se¬ 
rosa,  by  a  folding  of  the  primitive  extra-embryonal  blastoderm, 
is  rightly  considered  one  of  the  most  characteristic  features  of 
the  Hexapod  embryo.  The  envelopes  are  not,  however,  com¬ 
mon  to  all  insects.  An  amnion  is  completely  lacking  in  the 
Poduridae,^  and  consequently  the  extra-embryonal  blastoderm  in 
these  forms  is  strictly  comparable  to  the  corresponding  por¬ 
tion  of  the  blastoderm  in  Crustacea,  Myriopoda,  and  Arachnida. 
This  is  proved  by  the  fact  that  it  ultimately  forms  the  definitive 
dorsal  body-wall.  So  far  as  our  present  knowledge  extends, 
the  Apterygota  may  be  regarded  as  Hexapoda  Anamniota,  and 

1  Lemoine  (’87)  describes  a  cellular  “membrane  amniotique  ”  in  Anurophorus, 
but  he  does  not  represent  it  in  his  figures  and  did  not  study  it  in  section.  I 
therefore  incline  to  doubt  the  correctness  of  his  observation,  especially  as  I  can 
find  no  traces  of  a  cellular  envelope  in  the  Anurida  egg,  which  on  account  of  its 
size  is  a  far  more  favorable  egg  for  study  than  that  of  Anurophorus. 


6o 


WHEELER. 


[VOL.  VIII. 


placed  over  against  the  Pterygota,  which  are  characterized  by 
the  possession  of  an  amnion  (Ilexapoda  Amniota).  There  is 
a  gap  between  these  two  groups  of  insects  similar  to  the  gap 
between  the  amniote  and  anamniote  vertebrates.  Whether  it 
will  be  filled  by  the  future  study  of  such  orthopteroid  forms  as 
Machilis,  Lepisma  and  Forjictila  remains  to  be  seen.  For  the 
present  I  am  inclined  to  believe  that  the  amnion  first  made  its 
appearance  in  the  ancestral  Pterygota.  Even  if  it  be  contended 
that  the  amnion  was  once  present  in  the  Apterygota  and  subse¬ 
quently  lost,  its  origin  could  not  consistently  be  pushed  further 
back  than  the  Hexapoda,  since  this  envelope  is  lacking  in  the 
Myriopoda,  which,  there  is  reason  to  believe,  lie  in  the  direct 
line  of  descent.  The  proof  that  the  so-called  amnions  of  Peri- 
patiis,  Scorpions  and  Pseudoscorpions  are  the  homologues  of 
the  insect  amnion  is  not  forthcoming.  Judging  from  the  few 
descriptions  of  their  formation,  they  appear  to  have  arisen  in¬ 
dependently  within  their  respective  groups. 

Just  as  many  of  the  Pterygota  develop  only  rudiments  of 
wings  or  have  altogether  ceased  to  develop  these  organs  in  the 
adult  state,  so  the  embryos  of  the  Pterygota  in  some  cases 
develop  only  rudimental  envelopes  or  none  at  all.  It  is  reported 
that  the  amnion  is  lacking  in  the  Proctotrupid  Hymenoptera 
(Ayers,  ’84)  and  rudimental  in  Muscidae  (Kowalevsky,  ’86  ; 
Graber,  ’89)  and  viviparous  Cecidomyidae  (Metschnikoff,  ’66). 
Certain  ants  of  Madeira  are  incidentally  mentioned  by  Met¬ 
schnikoff  as  having  the  envelopes  represented  only  by  a  small 
mass  of  cells  in  the  dorsal  region.  The  absence  or  abortion 
of  the  amnion  is  almost  certainly  a  secondary  condition.  The 
Proctotrupidae  are  egg-parasites  and  undergo  an  extremely  ab¬ 
errant  embryonic  and  larval  development.  Both  these  and  the 
other  insects  mentioned  belong  to  groups  characterized  by 
high  specialization.  This  is  notably  the  case  with  the  ants 
and  with  the  Muscidae  which  show  considerable  aberration  in 
their  embryonic  and  larval  stages.  The  paedogenesis  of  the 
Cecidomyids  studied  by  Metschnikoff  stamps  them  also  as  ab¬ 
errant.  Moreover  the  embryos  of  other  Orthorrhaphous  Dip- 
tera  (Simulidae,  Chironomidae,  Tabanidae)  have  perfectly  normal 
envelopes. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  6 1 

Many  attempts  have  been  made  to  explain  the  origin  of  the 
amnion  in  insects.  It  first  appears  abruptly  and  fully  devel¬ 
oped  in  the  Orthoptera  just  as  the  vertebrate  amnion  appears 
abruptly  in  the  Reptilia.  One  school,  represented  by  Nus- 
baum  (’87)  and  v.  Kennel  (’85,  ’88),  regards  the  insect  amnion  as 
a  structure  of  high  phylogenetic  value  and  would  trace  it  to 
some  organ  in  the  lower  Arthropods  or  in  the  worms.  Ac¬ 
cording  to  another  view  advocated  by  Will  (’88)  and  myself 
(’89),  the  amnion  has  had  no  such  remote  phylogenetic  history, 
but  has  arisen  more  recently  in  response  to  certain  purely 
mechanical  conditions  of  development. 

Nusbaum  advances  the  opinion  that  the  cellular  envelopes 
of  the  insect  embryo  are  homologous  with  the  “dorsal  organ” 
of  Crustacea.  The  saddle-shaped  “dorsal  organ”  of  Ligia  and 
Onisctts  is  regarded  as  the  key  to  this  homology,  the  two  flaps 
which  clasp  the  sides  of  the  Isopod  embryo  being  equivalent 
to  undeveloped  amnioserosal  folds.  But  I  have  shown  in  the 
present  paper  that  the  indusium  of  Xiphidium  is  very  probably 
the  homologue  of  the  crustacean  “dorsal  organ,”  and  as  there 
is  besides  a  well  developed  amnion  and  serosa  in  Xiphidium, 
Nusbaum’ s  hypothesis  must  fall  to  the  ground.  His  assertion 
was  certainly  premature  that  the  “  deux  series  des  organes  aussi 
characteristiques  que  le  sont  I’organe  dorsal  et  les  enveloppes 
embryonnaires,  s’excluent  reciproquement  dans  les  deux 
groupes  des  Arthropodes,  c’est-a-dire  chez  les  Tracheates  et 
les  Crustaces.” 

So  far  as  the  insect  envelopes  are  concerned  v.  Kennel’s 
views  do  not  differ  essentially  from  Nusbaum’s.  He  likewise 
homologizes  the  crustacean  “dorsal  organ”  and  the  Poduran 
“micropyle”  with  the  Hexapod  amnion  and  serosa.  But  he 
goes  further  and  includes  under  the  same  homology  the 
amnion  of  Peripatus,  Scorpions  and  Chelifer  and  the  chitinous 
envelopes  of  Myriopods.  He  supposes  all  these  structures  to 
represent  remnants  of  the  annelid  trochophore.  I  feel  con¬ 
fident  that  he  has  jumbled  together  at  least  three  categories  of 
organs  which  cannot  be  regarded  as  homologous  inter  se,  viz. : 
(i)  the  series  of  structures  typically  represented  by  the  Crus¬ 
tacean  “dorsal  organ”;  (2)  the  cellular  envelopes  of  insects; 


62 


WHEELER. 


[VOL.  VIII. 


(3)  the  chitinous  cuticles.  As  stated  above,  the  amnions  of 
Peripatus  and  Scorpions  probably  also  represent  structures 
of  independent  origin  and  no  wise  homologous  with  the 
envelopes  of  insects.  It  is  perhaps  unnecessary  to  add  that 
the  reduction  of  all  these  structures  to  the  annelid  trochophore 
is  in  the  present  state  of  our  knowledge  little  more  than  a  wild 
guess. 

Graber  (’90)  has  criticized  the  view  advanced  by  Will  and 
myself,  that  the  insect  amnion  arose  by  an  invagination  of  the 
germ-band  like  that  of  some  Myriopods  (Geophilus),  His 
contention  is  certainly  in  great  measure  well-founded.  Still 
I  believe  that  it  does  not  affect  the  essential  point  of  the 
hypothesis  which  implies  that  the  amnioserosal  fold  is  the 
mechanical  result  of  a  local  induplication  of  the  blastoderm 
due  to  rapid  proliferation  in  a  single  layer  of  cells. 

Ryder  (’86)  has  sought  a  mechanical  explanation  for  the 
amnion,  and  although  his  paper  treats  mainly  of  the  vertebrate 
amnion,  he  evidently  implies  that  the  homonymous  envelope  of 
the  Insecta  had  a  similar  origin.  According  to  him  “the 
amnion  in  all  forms  has  arisen  in  consequence  of  the  forces 
of  growth  resident  in  the  embryo,  encountering  peripheral 
and  external  resistance  either  in  the  form  of  a  rigid  outer 
egg-shell  (zona  radiata)  or  decidua  reflexa,  or  even  the  walls 
of  the  uterine  cavity  itself,  supposing  of  course  that  a  large 
vesicular  blastoderm  containing  yolk  has  been  formed  by 
epiboly.” 

This  view  applies  with  little  alteration  to  the  Insecta.  There 
is  the  vesicular  one-layered  blastoderm  filled  with  yolk  and  the 
germ-band  arising  by  rapid  proliferation  at  one  point.  The 
resistance  of  the  yolk  being  less  than  the  external  resistance 
of  the  tightly  fitting  chorion  and  vitelline  membrane  on  the 
one  hand  combined  with  the  peripheral  resistance  of  the  extra- 
embryonal  blastoderm  on  the  other,  the  germ-band  is  forced  to 
invaginate.  This  invaginative  process  is  favored  by  the  dis¬ 
placement  of  yolk  during  its  liquefaction  and  absorption  by  the 
growing  embryo.  We  may  suppose  that  this  invagination 
which  results  in  the  formation  of  the  amnioserosal  fold, 
assumed  a  definite  and  specific  character  in  different  groups  of 
insects. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  63 

Conditions  similar  to  those  to  which  the  insect  germ-band  is 
subjected  during  its  younger  stages  are  often  present  in  the 
ova  and  young  of  other  animals,  and  would  be  expected  to  lead 
to  the  formation  of  structures  similar  to  the  insect  amnion. 
And  this  is  found  to  be  the  case.  A  hasty  glance  through  the 
animal  kingdom  at  once  suggests  a  number  of  parallel  instances : 
the  invagination  from  which  the  Cestode  head  develops  in  the 
Cysticercus  ;  the  similar  invagination  in  the  larval  Gordiid;  the 
origin  of  the  Nemertine  in  the  Pilidiitm;  the  formation  of  the 
definitive  trunk  in  Aulastoma,  according  to  Bergh  (’85);  the 
development  of  the  trunk  and  Scheitelplatte  in  Siptinctdusy 
according  to  Hatschek  (’84);  the  formation  of  the  young 
Spatangid  in  the  Pluteus,  according  to  Metschnikoff,  and  the 
somewhat  similar  conditions  in  the  development  of  the  An- 
tedon,  according  to  Barrois  (’88);  the  formation  of  the  trunk 
in  the  Actinotrocha  of  Phoronis  (E.  B.  Wilson  ’8l);  the  de¬ 
velopment  of  the  Polyzoan  within  the  statoblast  (Oka  ’91  ; 
Davenport  ’9i).  I  need  hardly  say  that  the  development  of 
the  amnion  and  serosa  in  vertebrates  is  a  strictly  analogous 
case.  A  case  still  more  to  the  point,  because  occurring  in 
the  Insecta,  is  the  formation  of  the  imaginal  disks.  In  this 
process  we  have  all  gradations  till  we  reach  the  extreme  in 
Miisca,  where  the  hollow  disks  whose  inner  walls  bud  forth 
the  imaginal  appendages  are  almost  completely  abstricted  from 
the  original  hypodermis.  The  resistance  of  the  chitinous  cuti¬ 
cle  of  the  larva  in  causing  the  invagination  of  the  disks  admits 
of  easy  observation.  It  certainly  cannot  be  claimed  that  in  all 
the  different  forms  here  enumerated  genetic  relationship  lies  at 
the  bottom  of  the  mutual  agreement  in  the  methods  of  form¬ 
ing  the  trunk  or  certain  organs.  On  the  contrary,  everything 
goes  to  show  that  these  similar  methods  in  widely  separated 
groups  have  been  independently  acquired  under  the  stress  of 
similar  developmental  conditions. 

Perhaps  the  most  difficult  point  to  explain  in  the  view  here 
advanced,  is  the  complete  abstriction  of  the  amnion  from  the 
serosa  in  nearly  all  insects.  It  is  more  natural  to  suppose 
that  the  inner  envelope  would  remain  continuous  with  the 
outer,  so  that  the  embryo  could  the  more  readily  be  everted 


64 


WHEELER, 


[VOL.  VIII. 


during  revolution.  The  only  explanation  I  have  to  offer,  will 
be  given  in  connection  with  a  discussion  of  the  movements  of 
the  germ-band.  In  that  connection  the  variations  in  the  devel¬ 
opment  and  amputation  of  the  envelopes  in  the  different  groups 
of  insects  may  also  be  treated  to  greater  advantage. 

2  The  Yolk. 

To  my  knowledge,  the  quantity  of  yolk  in  the  insect  egg 
has  not  been  made  the  subject  of  comparative  study.  It  has 
long  been  vaguely  stated  {vide  Brauer,  ’69  and  ’70)  that  the  eggs 
of  Ametabolous  insects  contain  relatively  more  yolk  than  the 
eggs  of  the  Metabola.  In  other  groups  of  animals  (Crustacea, 
Annelida,  Mollusca,  Vertebrata)  it  is  often  observed  that  ab¬ 
sence  of  yolk  is  correlated  with  free  larval  development,  while 
in  eggs  provided  with  an  abundance  of  yolk  the  larval  stages  are 
either  lacking  or  considerably  modified.  This  same  law  obtains 
also  in  the  Hexapoda,  though  it  can  hardly  be  formulated  so 
concisely  as  in  other  groups  of  animals.  And  this  is  not  sur¬ 
prising  when  we  stop  to  consider  that,  as  regards  complexity 
of  organization,  the  difference  between  the  simplest  insect 
larvae,  such  as  those  of  the  Muscidae  and  their  highly  special¬ 
ized  imagines,  is  far  from  being  as  great  as  the  differences 
between  the  trochophore  and  the  Annelid,  or  the  Nauplius  and 
the  crustacean. 

Beginning  with  the  Orthoptera  we  find  that  the  egg  is  pro¬ 
vided  with  an  abundance  of  yolk, — the  germ-band  when  first 
formed  in  most  cases  covers  only  a  very  small  portion  of  its 
surface,  and  when  it  reaches  its  maximum  length  before  revolu¬ 
tion  is  no  longer  than,  and  usually  not  so  long  as,  the  egg.^ 
The  period  of  embryonic  development  is  greatly  prolonged; 
most  of  the  species  are  monogoneutic  and  oviposit  in  the  fall, 
the  larvae  not  hatching  till  the  following  spring  or  summer. 
There  is  practically  no  metamorphosis. 

In  the  most  highly  metabolic  insects  (Muscidae)  on  the  other 
hand,  the  quantity  of  yolk  is  comparatively  limited.  The  germ- 
band  before  revolution  is  nearly  double  the  length  of  the  egg, 


1  To  this  rule  Gryllotalpa  seems  to  be  a  noteworthy  exception. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  65 


SO  that  the  head  and  tail  ends  nearly  meet.  Embryonic  de¬ 
velopment  is  completed  in  a  day,  and  the  larva  must  pass 
through  a  complex  metamorphosis  to  reach  the  imaginal  state. 

The  chasm  between  these  two  extremes  is  bridged  by  the 
less  metabolic  insects  (Coleoptera,  Neuropotera,  Lepidoptera, 
Hymenoptera,  etc.).  The  quantity  of  yolk  is  intermediate  be¬ 
tween  that  of  the  Orthopteran  and  Dipteran  egg.  The  germ- 
band,  like  that  of  the  Muscidae,  is  longer  than  the  egg  when  it 
reaches  its  full  length.  But  it  is  at  this  time  much  narrower 
than  the  yolk-mass,  whereas  in  the  Muscidae  it  embraces  nearly 
half  the  circumference  of  the  yolk.  The  larvae  usually  hatch 
after  a  period  of  ten  to  thirty  days  in  a  relatively  more  ad¬ 
vanced  stage  of  organization  than  Dipteran  larvae. 

It  is  probable  that  the  quality  of  the  yolk  is  also  an  impor¬ 
tant  factor  in  development.  The  yolk  of  the  Orthoptera  and 
Rhynchota  is  dense  and  resembles  that  of  the  crustacean  and 
Arachnid  egg,  while  the  yolk  of  the  Metabola  seems  to  have  a 
much  looser  molecular  structure.  Hence,  bulk  alone  is  no 
criterion  of  the  amount  of  yolk  in  an  insect’s  egg. 

The  view  here  advocated,  that  the  eggs  of  the  Ametabola 
contain  more  yolk  than  those  of  the  Metabola,  admits  of  some 
exceptions.  Thus  the  17-year  locust  {Cicada  septejtdeciiri) 
is  a  large  insect  with  incomplete  metamorphosis,  but  it  never¬ 
theless  produces  a  great  number  of  very  small  eggs.  This  is, 
however,  seen  to  be  a  greater  advantage  to  the  insect  than  the 
production  of  a  few  large  eggs,  when  we  consider  the  extremely 
long  period  of  larval  life  and  the  vicissitudes  to  which  the 
larvae  may  be  subjected  during  all  this  time.  Similarly,  Meloe 
angicsticollis  produces  a  great  number  of  very  small  eggs,  while 
the  eggs  of  the  smaller  beetles  {Doryphora,  e.gi)  are  much 
larger.  But  Meloe  is  a  parasite  form,  and  probably  only  a  few 
of  its  many  offspring  ever  succeed  in  gaining  access  to  the  eggs 
of  the  bee.  The  larvae,  as  shown  by  their  hypermetamorpho¬ 
sis,  are  subjected  to  very  varied  conditions,  and  this  would  still 
further  tend  to  reduce  the  number  of  successful  individuals. 
As  in  anemophilous  plants  many  germs  are  produced,  but  very 
few  are  destined  ever  to  prosper.  Many  other  exceptions  to 
the  general  rule,  like  these  two,  are  probably  due  to  habits 


66 


WHEELER. 


[VOL.  VIII. 


which  necessitate  the  production  of  a  great  number  of  ova  at 
the  expense  of  their  size.  The  opposite  exception  occurs  in 
the  parasitic  Pupipara,  where  the  nourishment  of  the  single 
larva  within  the  parent  is  equivalent  to  the  production  of  a  large 
yolk-laden  egg.^ 

The  question  naturally  arises:  Were  the  eggs  of  the  prim¬ 
itive  Insecta  poor  or  rich  in  yolk }  As  all  the  evidence  of  com¬ 
parative  anatomy,  embryology  and  paleontology  goes  to  show 
that  the  Metabola  are  the  more  recent,  the  Ametabola  the 
more  ancient  forms,  we  are  justified  in  maintaining  that  prim¬ 
itive  insects,  or  at  any  rate  the  primitive  Pterygota  supplied  . 
their  eggs  with  a  considerable  quantity  of  yolk.  At  first  sight 
the  Apterygota,  which  have  holoblastic  eggs,  would  seem  to 
constitute  a  serious  obstacle  to  this  view,  but  it  must  be 
remembered  that  total  cleavage  is  not  necessarily  a  criterion  of 
paucity  of  yolk  (witness  Arachnida,  Crustacea,  and  Myriopoda). 
Furthermore,  the  eggs  of  some  Thysanura,  Amirida,  e.g.  are 
provided  with  an  abundance  of  yolk.  Holoblastic  cleavage  in 
this  group  is  probably  a  Myriopod  trait,  as  was  long  ago  sug¬ 
gested  by  Metschnikoff  (’74).  We  might  perhaps  conclude 
that  the  superficial  type  of  cleavage,  like  the  embryonic  en- 

1  The  differences  between  the  eggs  of  different  insects  with  respect  to  the 
amount  of  yolk  is  systematically  disregarded  by  Graber  (’90).  This  is  shown  by 
his  classification  of  germ-bands  as  microblastic  and  macroblastic,  brachyblastic  and 
tanyblastic.  These  distinctions  are  readily  shown  to  be  distinctions  in  the  amount 
of  yolk  and  not  in  the  germ-band.  Thus  the  just-established  germ-bands  of  the 
Saltatory  Orthoptera  appear  to  be  very  small  because  the  eggs  contain  an  enor¬ 
mous  quantity  of  yolk;  while  the  germ-band  of  the  Muscidae  appears  correspond¬ 
ingly  large  on  account  of  the  small  quantity  of  yolk.  The  amount  of  yolk 
fluctuates  even  within  the  limits  of  the  single  orders  so  that  the  newly-formed  germ- 
bands  appear  to  differ  in  length  more  than  they  really  do.  In  the  Orthoptera  we 
have  the  following  series  in  which  the  amount  of  yolk  decreases,  the  germ-band  in 
consequence  appearing  to  increase:  Melanophis,  Mantis,  QLcanthns,  GrylhiSy 
Xiphidium,  Blatta,  (?)  Gryllotalpa. 

Graber’s  further  classification  of  germ-bands  as  orthoblastic  and  ankyloblastic, 
or  straight  and  curved,  is  equally  artificiaL  In  the  great  majority  of  cases  the 
shape  of  the  germ-band  depends  upon  the  yolk  surface  on  which  it  arises,  or  over 
which  it  happens  to  grow.  The  uselessness  of  such  a  classification  is  also  shown 
in  the  case  of  Xiphidium  and  Orchelimum,  where  the  just-established  germ-band 
is  straight,  but  becomes  curved  in  passing  to  the  dorsal  surface,  and  thereupon 
again  becomes  straight.  To  which  of  Graber’s  classes  does  this  germ-band 
belong  t 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


velopes  and  the  wings,  originated  in  the  ancestral  Pterygota. 
But  Lemoine  (’87)  claims  that  the  segmentation  of  the  Poduran 
AimrophortLS  laricis  approaches  the  superficial  type,  so  that 
this  latter  may  have  had  a  still  more  remote  origin.  It  is, 
however,  hopeless  to  speculate  on  this  subject  till  the  eggs  of 
many  more  Thysanura  and  Myriopoda,  including  the  Symphyla, 
have  been  studied. 

The  relations  of  yolk-quantity  to  the  movements  of  the 
embryo  will  be  considered  in  the  following  paragraphs. 

3.  Blastokinesis. 

According  to  Hallez  (’85  and  ’86)  ‘‘  La  cellule-oeuf  possMe 
la  meme  orientation  que  I’organisme  maternel  qui  Ta  produite: 
elle  a  un  pole  cephalique  et  un  pole  caudal,  un  cote  droit  et 
un  cote  gauche,  une  face  dorsale  et  une  face  ventrale;  et  ces 
differentes  face  de  la  cellule-oeuf  coincident  aux  faces  cor- 
respondantes  de  Tembryon.”  This  law  was  founded  on  a  study 
of  the  eggs  of  Periplajieta^  Hydrophilus  and  Locicsta,  but  it 
finds  full  support  in  the  descriptions  and  figures  of  all  investi¬ 
gators  of  insect  development.^  My  own  observations,  based  on 
some  thirty  different  insects,  accord  perfectly  with  those  of 
Hallez. 

In  most  eggs  the  cephalic  and  caudal  poles  are  readily  dis¬ 
tinguishable,  the  micropyle  being  usually  located  at  or  near 
the  former.  In  exceptional  cases,  however,  it  is  located  at  the 
caudal  pole.  There  is  frequently  a  slight  flexure  in  the  longi¬ 
tudinal  axis  of  the  egg,  foreshadowing  the  dorsal  and  ventral, 
and  consequently  also  the  lateral  regions  of  the  mature  em¬ 
bryo.  The  more  nearly  the  egg  approaches  the  spherical  form, 
as  in  certain  Lepidoptera  and  Coleoptera  and  in  the  Tricho- 
ptera,  the  more  obscure  become  the  relations  of  the  egg-sur¬ 
faces  to  the  body-surfaces  of  the  mature  embryo.  There  is, 
however,  every  reason  to  suppose  that  these  relations  still  exist. 

The  practical  value  of  Hallez’  law  was  shown  in  studying 
the  Xiphidiinn  egg  ;  all  the  movements  of  the  germ-band  could 

1  The  only  exception  is  Ayers,  who  was  undoubtedly  mistaken  in  regard  to  the 
orientation  of  the  young  CEcanthus  embryo. 


68 


WHEELER. 


[VOL.  VIII. 


at  once  be  referred  to  the  axis  of  the  mature  embryo.  When 
the  eggs  of  other  insects  are  oriented  in  the  same  manner,  it  is 
seen  that  the  germ-band  invariably  arises  on  the  ventral  sur¬ 
face  of  the  yolk  with  its  procephaleum  directed  towards  the 
cephalic,  and  its  tail  towards  the  caudal  pole.  No  matter  what 
positions  it  may  subsequently  assume,  it  always  returns  to  its 
original  position  before  hatching.  Frequently  the  germ-band, 
when  newly  formed,  lies  nearer  the  lower  than  the  upper  pole 
{Calopteryx^  OEca^ttJms^  Siag7nomantzs,  Hydrop/iiltis,  etc.).  The 
usual  movements  are  very  simple  ;  from  a  position  of  rest  on 
the  ventral  surface  of  the  egg,  the  germ-band  moves  through 
an  arc  till  its  body  is  completely  inverted.  Then  it  rests  and 
again  passes  back  through  the  same  arc  to  its  original  position 
on  the  ventral  yolk.  These  movements  may  be  compared  to 
the  single  vibration  of  a  pendulum.  The  ascending  movement 
I  shall  designate  as  anatrepsis^  the  descending  as  katatrepsis, 
the  intervening  resting  stage  as  the  diapause.  The  general 
term  blastokinesis  may  be  used  to  include  all  the  oscillatory 
movements  of  the  germ-band. 

Inasmuch  as  the  germ-bands  in  other  Arthropods  (Crustacea, 
Myriopoda,  Arachnida,  and  Thysanura)  exhibit  no  movements 
comparable  to  those  of  the  lower  Pterygota,  and  since,  more¬ 
over,  the  insect  germ-band  is  formed  in  exactly  the  same 
manner  as  that  of  other  Arthropods  and  ultimately  returns  to 
its  original  position,  no  matter  what  oscillations  may  intervene, 
it  is  safe  to  infer  that  blastokinesis  has  been  acquired  within 
the  Hexapod  and  probably  even  within  the  Pterygote  group. 
We  may  also  infer  from  the  intimate  relations  of  envelope- 
formation  to  blastokinesis  in  most  forms,  that  both  of  these 
processes  arose  at  about  the  same  time. 

No  attempt  has  been  made  to  account  for  the  origin  of  blasto¬ 
kinesis.  It  has  occurred  to  me  that  it  may  be  due  to  causes  of 
a  purely  physiological  nature.  The  eggs  of  the  primitive  Pter¬ 
ygota  were,  as  I  have  attempted  to  show,  provided  with  a  con¬ 
siderable  amount  of  food  yolk.  Like  their  modern  descendants 
they  were  probably  also  invested  with  dense  chitinous  envelopes. 
These  must  render  the  respiration  of  the  embryo  difficult  as 
compared  with  embryonic  respiration  in  annelids,  mollusks  and 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


vertebrates,  or  even  as  compared  with  _the  Crustacea,  which 
usually  have  much  thinner  envelopes  than  insect  eggs.  Special 
provision  is  also  made  in  many  of  the  Crustacea  for  aerating  the 
eggs.  Now  the  cells  of  the  rapidly  growing  insect  embryo  not 
only  absorb  and  metabolize  the  yolk  but  also  give  off  a  certain 
amount  of  waste  matter.  That  this  is  not  wholly  of  a  gaseous 
nature  is  seen  in  older  embryos  which  have  considerable 
accumulations  of  uric  salts  in  the  blood  corpuscles  and  fat- 
body.  Waste  products  are  undoubtedly  given  off  during  the 
stages  preceding  anatrepsis,  and  probably  permeate  the  yolk  in 
the  immediate  neighborhood  of  the  germ-band.  As  the  oxida¬ 
tion  of  these  waste  products  is  very  probably  retarded  by  slug¬ 
gish  transpiration,  and  as  growth  under  such  conditions  would 
be  seriously  impeded,  we  may  suppose  that  the  embryo  has 
acquired  the  habit  of  moving  to  another  part  of  the  egg  where 
the  yolk  is  as  yet  unpolluted.  Here  it  grows  apace  till  the 
surrounding  yolk  is  again  charged  with  excreta.  Growth  is 
then  temporarily  suspended  and  the  embryo  moves  back  to  the 
ventral  surface.  The  embryo  reaches  a  considerable  size  be¬ 
fore  katatrepsis,  so  that  its  rotation  must  cause  a  considerable 
circulation  in  the  yolk  bodies.  This  would  also  serve  to  aerate 
the  yolk  and  to  bring  fresh  pabulum  in  contact  with  the 
assimilating  cells  of  the  embryo.  It  may  also  be  noted  that 
in  many  insects  the  movements  set  in  at  critical  periods  of 
growth.  Thus  in  Xiphidium  anatrepsis  occurs  during  the 
addition  of  new  segments,  and  in  many  other  forms  it  im¬ 
mediately  precedes  the  formation  of  new  segments.  In  the 
Orthoptera,  katatrepsis  usually  occurs  in  the  spring  and  is  the 
signal  for  a  decisive  advance  in  the  development  of  the  heart, 
sexual  organs,  compound  eyes,  etc.  During  this  period,  also, 
the  abortion  of  such  rudimental  structures  as  the  pleuropodia, 
abdominal  appendages  and  envelopes  seems  to  be  hastened.  In 
short,  the  whole  process  of  katatrepsis,  at  least  in  Xiphidium^ 
has  the  aspect  of  rejuvenescence.  It  will  be  remembered  that 
the  amnion  is  formed  just  before  or  during  anatrepsis.  It  is 
probable  that  the  complete  abstriction  of  this  envelope  from 
the  serosa  is  a  device  for  favoring  the  movements  of  the  embryo. 
The  germ-band  is  thereby  set  adrift  on  the  yolk  and  enabled  to 


70 


WHEELER. 


[VOL.  VIII. 


migrate  to  some  other  surface.  This,  of  course,  necessitates  a 
secondary  union  of  the  envelopes  previous  to  katatrepsis. 

The  hypothesis  set  forth  in  the  preceding  paragraphs  is  also 
supported  indirectly  by  the  fact  that  in  the  eggs  of  the  Meta- 
bola  which  are  less  abundantly  provided  with  yolk  than  the 
eggs  of  the  Ametabola,  blastokinesis  is  either  faint  or  wanting. 
Aeration  would  be  much  less  necessary  in  such  small  eggs. 
The  lengthening  and  shortening  movements  seen  in  the 
embryos  of  the  Metabola  as  well  as  in  those  of  the  Ametabola 
may  suffice  to  keep  the  yolk  circulating.  The  Lepidopteran 
germ-band,  it  is  true,  exhibits  movements,  but  the  eggs  of 
these  insects  are  laid  in  exposed  situations  and  provided  with 
unusually  thick  envelopes,  so  that  the  movements  of  the  em¬ 
bryo,  though  differing  widely  from  the  typical  blastokinesis  of 
lower  forms,  have  perhaps  been  independently  acquired  for  a 
similar  purpose. 

I  had  intended  to  give  a  comparative  description  of  blasto¬ 
kinesis  in  the  different  orders  of  insects  but  as  the  known 
facts  have  been  recently  summarized  in  a  masterly  manner 
by  Korschelt  and  H eider  (’92)  I  shall  confine  my  remarks 
mainly  to  the  Orthoptera.  Although  Graber,  Ayers  and 
others  have  studied  representatives  of  this  very  important 
group,  they  have  given  but  fragmentary  and  often  inaccurate 
accounts  of  the  relations  of  the  embryo  to  the  yolk-mass  at 
different  periods  of  development. 

I  may  begin  my  account  with  the  Saltatoria  which  comprise 
the  three  families  Gryllidae,  Locustidse  and  Acrididae.  As 
representatives  of  the  first,  Gryllus  luctuosus  and  CEcmithus 
niveiLS  were  studied.  In  both  of  these  insects  as  was  pointed 
out  at  p.  42  the  germ-band  arises  on  the  ventral  surface  of 
the  yolk  near  the  caudal  pole.  During  the  formation  of  the 
envelopes  anatrepsis  sets  in  and  carries  the  germ-band  to  the 
dorsal  surface  where  it  rests  through  the  winter  in  an  inverted 
position  with  its  head  directed  to  the  caudal  and  its  tail  to  the 
cephalic  pole.  In  the  spring  the  envelopes  over  the  head  end 
first  fuse  and  then  rupture  ;  the  embryo  is  thereupon  everted 
and  during  katatrepsis  passes  around  the  caudal  pole  to  regain 
its  upright  position  on  the  ventral  yolk.  The  envelopes  during 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


71 


this  process  are  stripped  back  over,  and  finally  drawn  into  the 
yolk,  where  they  undergo  dissolution  when  the  body  walls  have 
met  in  the  median  dorsal  line.  The  defects  in  Ayers’  descrip¬ 
tion  of  CEcantJms  (’84)  were  pointed  out  at  p.  43. 

Gryllotalpa.,  the  only  other  Gryllid,  which  has  been  studied, 
seems  to  differ  considerably  from  Grylhts  and  QLcantJms.  Ex¬ 
amination  of  Korotneff’s  figures  (’85)  shows  that  this  difference 
is  probably  more  apparent  than  real.  In  his  surface  views, 
there  is  a  wide  gap  between  his  Fig.  2,  representing  the  egg  in 
a  preblastodermic  stage,  and  his  Fig.  3,  representing  quite  an 
advanced  embryo.  One  is  thus  left  without  any  guide  to  the 
exact  relation  of  the  just-established  germ-band  to  the  yolk- 
surfaces.  Korotneff’s  defective  account  of  the  formation  of 
the  germ-layers  would  seem  to  show  that  he  did  not  study  these 
early  stages  closely.  It  is  obvious  that  Gryllotalpa  is  blasto- 
kinetic  both  from  Korotneff’s  statement  that  the  embryo 
moves  during  revolution  and  from  his  figures  5,  7,  and  8,  but 
the  exact  nature  of  the  process  is  not  clear.  The  possibility 
of  the  embryo’s  passing  to  the  opposite  surface  of  the  egg  is 
not  precluded  by  the  conditions  seen  in  Figs.  7  and  8.  Judg¬ 
ing  from  Grylhis  and  CEcanthus  I  am  inclined  to  think  that  the 
embryo  exhibits  both  ana-  and  katatrepsis,  but  that  Korotneff 
has  overlooked  the  former  and  misinterpreted  the  latter 
movement. 

In  the  Locustidae,  as  represented  by  Xiphidium  and  Orcheli- 
miim,  we  find  a  modification  of  the  blastokinetic  process  ob¬ 
served  in  Grylhis.  Instead,  however,  of  arising  near  the  caudal 
pole,  the  germ-band  is  formed  on  the  middle  of  the  ventral 
surface,  and  instead  of  passing  around  the  caudal  pole  during 
anatrepsis  it  passes  through  the  yolk  as  if  to  reach  the  dorsal 
surface  by  a  shorter  path.  Katatrepsis  is  essentially  the  same 
as  in  the  Gryllidae,  the  embryo  passing  around  the  caudal  pole. 
This  lack  of  coincidence  in  the  anatreptic  and  katatreptic  paths 
is  one  of  the  most  striking  peculiarities  of  Locustid  develop¬ 
ment;  since  it  is  known  to  occur  in  no  other  insect.  It  is 
probable  that  the  anatreptic  embryo  originally  passed  around 
the  lower  pole,  but  that  owing  to  the  formation  of  the  embryo 
higher  up  on  the  ventral  surface,  and  perhaps  also  to  an  acqui- 


72 


WHEELER. 


[VOL.  VIII. 


sition  of  yolk  at  the  lower  pole,  this  movement  has  been 
deflected. 

Melanophis  femiir-rtibriLin  was  studied  as  a  representative  of 
the  Acrididae.  The  germ-band  is  formed  very  near  the  caudal 
pole  of  the  egg,  but  still  on  the  convex  ventral  surface.  During 
the  formation  of  the  envelopes  the  posterior  end  of  the  body 
grows  around  the  pole  onto  the  dorsal  surface,  while  its  head 
remains  fixed  at  the  pole.  It  is  not  until  the  germ-band  has 
reached  a  stage  corresponding  to  Stage  F.  in  Xiphidium  that 
its  head  leaves  the  pole  and  the  whole  body  moves  upward  on 
the  dorsal  surface.  It  soon  comes  to  a  standstill  and  passes 
the  winter  in  this  inverted  position.  In  the  spring  it  moves 
back  around  the  lower  pole  and,  like  the  Gryllid  and  Locustid 
embryo  in  a  corresponding  stage,  proceeds  to  lengthen  and  en¬ 
velop  the  yolk  till  its  head  reaches  the  cephalic  pole. 

Packard  (’83)  seems  to  have  been  the  first  to  study  the  devel¬ 
opment  of  Acridians  {Melanophis  spretns  and  M.  atlanis).  But 
he  had  no  conception  of  the  true  relations  of  the  embryo  to  the 
yolk,  as  is  shown  by  his  Fig.  i,  PI.  XVII,  where  the  egg  is 
depicted  with  the  micropylar  end  uppermost.  Leuckart  (’55) 
long  ago  showed  that  the  Acridian  micropyle  is  located  at  the 
caudal  pole.  If  the  egg  figured  by  Packard  be  inverted,  it  will 
represent  the  embryo  on  the  point  of  undergoing  katatrepsis. 

The  same  error  is  committed  by  Graber  in  his  accounts 
of  Stenobothrns  variabilis  (’88,  ’90).  Misled,  like  Packard,  by 
the  position  of  the  micropyle,  he  has  mistaken  the  caudal  for 
the  cephalic  pole.  To  mean  anything  his  figures  must  be  in¬ 
verted.  As  I  have  not  yet  studied  the  later  stages  of  Melan- 
opliis  in  section,  I  will  not  attempt  to  describe  the  details  of 
katatrepsis.  Graber  claims  to  have  observed  that  the  pleural 
ectoderm,  where  it  passes  into  the  amnion  proliferates  a  thin 
cell-lamella  to  form  the  dorsal  wall,  while  the  amnion  remains 
intact  and  still  covers  the  ventral  face  of  the  embryo.  This 
account  is  not  substantiated  by  his  figures  (i  and  2,  PI.  I). 
The  two  thin  cell-lamellae  extending  over  the  dorsal  surface 
have  every  appearance  of  being  the  walls  of  the  heart,  and 
therefore  mesodermal,  although  it  is  difficult  to  see  how  this 
organ  could  be  so  completely  formed  in  so  early  a  stage.  As 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


Graber  has  paid  no  attention  to  the  movements  of  the  Steno- 
bcthriis  embryo,  and  as  he  most  assuredly  has  not  demonstrated 
from  a  careful  study  of  the  later  stages  that  the  lamella  in 
question  is  really  converted  into  the  dorsal  wall,  I  cannot 
attribute  much  value  to  his  observation. 

The  foregoing  observations  go  to  show  that  the  blastokinetic 
processes  are  essentially  the  same  throughout  the  suborder 
Saltatoria.  Each  family  presents  certain  deviations  from  the 
type,  which  is  probably  most  closely  adhered  to  in  the  Gryllidae. 
Anatrepsis  is  aberrant  in  the  Locustidae,  while  the  Acrididae 
are  aberrant  in  the  tardy  separation  of  the  procephaleum  from 
the  lower  pole.  Notwithstanding  these  deviations  the  Salta¬ 
toria  form  a  clearly  circumscribed  group  embryologically  as 
well  as  anatomically,  and  were  it  not  for  Gryllotalpa  would  be 
separated  by  a  wide  gap  from  all  other  Orthoptera.  Gryllo¬ 
talpa  is  a  generalized  form,  as  Brauer  has  pointed  out  from 
a  study  of  its  anatomical  peculiarities  (’86),  and  his  conclusions 
are  to  some  extent  substantiated  by  the  large  size  of  the  germ- 
band  as  compared  with  the  yolk  mass. 

In  the  Cursoria,  as  represented  by  Blatta  germanica,  move¬ 
ments  of  the  embryo  are  far  less  apparent.  The  germ-band 
never  leaves  the  ventral  surface,  on  the  middle  of  which  it  first 
appears.  I  have  shown,  nevertheless  (’89,  text-figures,  p.  348), 
that  it  moves  down  the  yolk  after  the  rupture  of  the  envelopes 
till  its  tail  reaches  the  lower  pole.  The  tail  then  remains  sta¬ 
tionary,  while  the  head  gradually  rises  to  the  cephalic  pole  as 
the  body  walls  develop  and  invest  more  and  more  of  the  yolk. 
Slight  as  are  these  movements,  they  nevertheless  recall  the 
blastokinesis  of  the  Saltatoria.  I  would  regard  the  movement 
of  the  whole  Blatta  embryo  towards  the  caudal  pole  as  ana- 
treptic  ;  katatrepsis  is  probably  represented  only  by  the  up¬ 
ward  growth  of  the  embryo.  The  very  late  occurrence  of  the 
former  movement  may  be  due  to  its  rudimental  character,  since 
it  is  too  weak  to  carry  the  germ-band  around  the  caudal  pole. 

Few  observations  have  been  published  on  the  relations  of 

♦ 

the  embryo  to  the  yolk  in  the  Gressoria.  In  Mantis,  as  I  have 
shown,  the  germ-band  when  first  formed  lies  somewhat  nearer 
the  posterior  than  the  anterior  pole.  The  embryo  never  leaves 


74 


WHEELER, 


[VOL.  VIII. 


the  ventral  surface  of  the  egg,  but  whether  or  not  it  exhibits 
any  traces  of  blastokinesis  my  limited  material  will  not  enable 
me  to  decide,  and  Graber  (’77),  Bruce  (’86),  and  Viallanes 
(’90®,  ’90^^),  have  contributed  no  observations  bearing  on  this 
point.  It  is  clear,  nevertheless,  that  in  its  development.  Mantis 
resembles  Blatta  more  closely  than  either  of  these  forms  re¬ 
semble  the  Saltatoria.  This  merely  confirms  the  view  which 
has  long  been  held  respecting  the  affinities  of  the  Blattidae 
and  Mantidae.  From  the  structural  similarity  of  the  Phas- 
midae  and  Mantidae  we  may  venture  to  infer  a  similarity  of 
embryonic  development. 

It  thus  appears  that  the  Orthoptera  are  clearly  separable 
into  two  groups  —  the  Saltatoria  on  the  one  hand  and  the 
Gressoria  and  Cursoria  on  the  other.  The  Saltatoria  are 
decidedly  blastokinetic  whereas  the  non-saltatory  forms  re¬ 
tain  only  faint  reminiscences  of  blastokinesis  {Blatta).  I 
am  inclined  to  believe  that  primitive  embryological  features 
have  been  preserved  more  faithfully  in  the  Saltatoria  than 
in  other  Orthoptera.  That  the  habits  of  oviposition  are  more 
primitive  in  this  group  is  shown  by  Brongniart’s  discovery  of  a 
fossil  Blattid  provided  with  an  ovipositor  (’89).  Moreover,  several 
features  in  the  development  of  the  Saltatoria  show  great  con¬ 
servatism,  e.g.  the  retention  of  the  indusium  in  the  Locust- 
idae,  the  order  in  which  the  metameres  arise,  and  the  myriopod- 
like  habitus  of  the  XipJiidiiim  embryo  in  Stage  D. 

Not  only  does  a  study  of  the  Saltatoria  throw  light  on  the 
development  of  other  Orthoptera,  but  it  brings  the  order  into 
closer  union  with  the  Odonata  and  Rhynchota.  The  blasto¬ 
kinesis  of  the  Gryllidae  agrees  closely  with  that  of  the  Hydro- 
corisa  among  the  Hemiptera  —  eg.  Corixa,  as  described  by 
Metschnikoff  (’66).  Ra^iatra  and  ZaitJia  will  bear  even  a  closer 
comparison  with  the  Gryllidae.  In  the  much  elongated  egg  of 
the  former,  which  has  the  cephalic  pole  marked  by  the  pair  of 
diverging  pneumatic  threads,  the  germ-band  arises  as  usual  on 
the  ventral  surface  with  its  head  directed  upwards.  As  the 
envelopes  develop  it  passes  around  the  lower  pole  and  finally 
assumes  an  inverted  position  on  the  dorsal  surface.  During 
katatrepsis  it  returns  over  the  same  path.  The  inclusion  ob- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


servable  in  Corixa  and  probably  also  in  Ranatra^  of  a  small 
quantity  of  yolk  between  the  caudal  amnion  and  the  overlying 
serosa  when  the  embryo  first  passes  to  the  dorsal  surface,  is 
often  observed  in  the  Saltatoria.  It  is  no  great  step  to 
pass  from  the  conditions  seen  in  the  Hydrocorisa  to  the 
“  entoblastic  ”  condition  of  other  Hemiptera  {Pediculus,  Aphis, 
Cicada)  and  the  Odonata  {Calopteryx) ,  where  the  germ-band 
instead  of  passing  to  the  dorsal  yolk  during  anatrepsis,  comes 
to  lie  in  the  middle  of  the  yolk,  or  even  near  the  ventral 
surface  (Pj/j'-rkocoris) .  The  Thysanoptera,  as  may  be  in¬ 
ferred  from  Jordan’s  brief  statement  (’88),  the  Corrodentia 
(Mallophaga)  according  to  Melnikow  (’69),  and  the  Psocidae, 
according  to  Packard  (’84),  are  also  referable  to  the  ‘‘ento- 
blastic”  type.  Concerning  the  embryonic  development  of  the 
Plecoptera  and  Dermaptera  nothing  is  known. 

So  far  only  the  Homomorpha  have  been  considered.  The 
eggs  of  the  Heteromorpha,  as  I  have  attempted  to  show,  con¬ 
tain  less  yolk.  Blastokinesis  is  nearly  or  quite  lost  in  this 
more  recent  group,  a  fact  that  perhaps  indirectly  tells  in  favor 
of  my  view  that  the  movements  of  the  embryo  have  been  ac¬ 
quired  for  the  purpose  of  ventilating  the  yolk  and  supplying 
the  growing  embryo  from  time  to  time  with  fresh  pabulum. 
The  transition  to  the  Heteromorpha  is  probably  represented  by 
the  Ephemeridea.  According  to  Burmeister’s  account  of  the 
development  of  Palingenia  horaria  (I  quote  from  Zaddach,  ’54) : 
^‘am  dritten  Tage,  nachdem  das  Ei  gelegt  war,  hatte  sich  der 
Keimstreif  gebildet,  der  zungenformig  war,  und  sich  liber  zwei 
drittel  der  Eilange  erstreckte,  also  in  Form  und  Ausdehnung 
ganz  dieselben  Verhaltnisse  zeigte,  wie  im  Phryganidenei.” 
This  may,  perhaps,  be  taken  to  indicate  that  the  Ephemeridea 
exhibit  no  blastokinesis  ;  but  the  subject  requires  urgent 
investigation. 

Among  the  Heteromorpha  it  is  especially  the  Coleoptera 
which  still  show  distinct  though  abortive  movements  of  the 
germ-band.  Hydrophihis  may  be  taken  as  an  example.  As 
may  be  seen  from  Holder’s  figures,  the  germ-band  forms  on  the 
lower  ventral  surface  of  the  egg.  As  it  grows  in  length,  and 
the  amnion  is  formed,  the  tail  curls  around  the  caudal  pole  on- 


76 


WHEELER, 


[VOL.  VIII. 


to  the  dorsal  surface,  but  soon  separates  from  the  serosa  so 
that  a  small  amount  of  yolk  is  enclosed  between  the  two  en¬ 
velopes.  Later  the  yolk  is  expelled  from  this  region  and  the 
envelopes  become  applied  to  each  other.  A  true  movement 
then  sets  in  and  carries  the  anterior  portion  of  the  germ-band 
forward  up  the  ventral  surface  till  the  procephaleum  overlaps 
the  cephalic  pole  {Cf.  Heider’s  figures,  4  6  J  a  and  9, 

PI.  II.  (’89).  A  certain  similarity  of  these  movements  to  those 
exhibited  in  Blatta  leads  me  to  believe  that  they  represent 
a  weakened  blastokinesis. 

Whether  or  not  similar  movements  occur  in  the  other  so- 
called  “  ectoblastic  ”  forms  (Diptera,  Hymenoptera,  Siphonap- 
tera,  Neuroptera,  Trichoptera)  cannot  be  decided  at  present. 
If  such  movements  occur  at  all  they  are  probably  exceedingly 
weak. 

As  stated  above,  the  Lepidoptera  have  developed  embryonic 
movements  peculiar  to  themselves.  In  all  the  members  of  the 
order  hitherto  studied,  the  germ-band  arises  on  the  ventral  sur¬ 
face  of  the  egg,  and  its  envelopes  are  formed  while  it  is  still 
in  this  position.  As  development  proceeds  the  convex  ventral 
surface  of  the  germ-band,  with  its  adherent  amnion,  moves  back 
from  the  ventral  serosa  and  soon  comes  to  lie  in  the  middle  of 
the  yolk.  Hereupon  the  ventral  surface  of  the  embryo  be¬ 
comes  concave,  and  its  dorsal  surface  is  applied  to  the  dorsal 
serosa.  I  have  already  remarked  that  this  movement  of  the 
embryo  may  have  been  independently  developed  for  the  same 
purely  physiological  purposes  as  blastokinesis  in  the  Homo- 
morpha.  The  fact  that  the  movement  is  represented  in  the 
Trichoptera  only  by  the  change  in  flexure  of  the  longitudinal 
embryonic  axis,  would  seem  to  indicate  that  it  has  been 
acquired  since  the  Lepidoptera  diverged  from  the  Trichopteroid 
ancestor. 

Graber  (’9o)  has  recently  made  the  interesting  discovery  that 
the  Phytophagous  Hymenoptera  closely  resemble  the  Lepidop¬ 
tera  in  the  movements  of  the  embryo  and  in  the  amputation  of 
the  envelopes.  This,  taken  together  with  the  striking  resem¬ 
blance  between  the  eruciform  larvae  of  the  two  groups,  appears 
to  point  to  a  closer  relationship  than  has  usually  been  claimed. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


While  studying  the  movements  of  the  embryo  and  the  for¬ 
mation  of  the  envelopes  in  the  different  orders  and  families  of 
insects,  with  a  view  to  testing  the  current  classification,  which 
is  the  outcome  of  a  great  amount  of  comparative  anatomical 
and  paleontological  work,  I  have  been  especially  impressed 
with  two  facts:  First,  the  embryological  data  in  no  wise  conflict 
with  the  generally  accepted  classification  of  Brauer.  The  de¬ 
velopmental  variations  within  limited  groups  are  never  greater 
than  the  post-embryonic  differences  in  the  members  of  the 
same  groups.  Usually  there  is  great  uniformity  in  embryo- 
logical  development  between  systematically  allied  insects  of 
the  same  order  ;  the  wide  gaps  usually  occur  between  the 
orders  just  where  gaps  have  long  been  pointed  out  by  com¬ 
parative  anatomy  and  paleontology.  Second,  developmental 
differences  between  members  of  different  allied  families  of 
Orthoptera  are  greater  than  the  differences  between  remotely 
related  families  in  more  recent  orders.  For  example,  the  dif¬ 
ferences  between  a  Locustid  and  an  Acridian  or  a  Locustid 
and  a  Gryllid  embryo,  or  between  any  of  the  Saltatoria  and 
the  Blattidae,  or  Mantidae,  are  greater  than  the  differences  be¬ 
tween  an  embryo  Hydrophilid  and  a  Chrysomelid,  a  Tabanid 
and  a  Chironomid,  or  a  Bombycid  and  a  Shingid.  Frequently, 
it  is  true,  the  differences  between  the  extremes  in  the  higher 
orders  are  considerable,  as  between  the  Tenthredinidae  and  the 
Proctotrupidae  among  Hymenoptera,  or  the  Chironomidae  and 
Muscidae  among  Diptera.  If  any  conclusions  bearing  on  classi¬ 
fication  can  be  drawn  from  the  few  embryological  data  which 
I  have  collected,  they  refer  to  the  ordinal  value  of  the  vari¬ 
ous  Orthopteran  families.  It  would  appear  that  these  groups 
have  really  more  than  family  value.  They  are  older  than  the 
families  of  more  recent  groups,  and  therefore  exhibit  greater 
divergence.  The  Rhynchota  will  probably  be  found  to  present 
conditions  similar  to  the  Orthoptera.  There  are  certainly  more 
considerable  differences  between  the  embryos  of  such  forms  as 
Pyrrhocoris  and  Ranatra  than  there  are  between  the  embryos 
of  widely  separated  families  among  the  Coleoptera. 


78 


WHEELER. 


[VOL.  VIII. 


4.  The  Elimination  of  the  Embryonic  Envelopes. 

Anatrepsis  and  katatrepsis  in  the  lower  insect  orders,  or  the 
completion  of  the  envelopes  and  their  rupture  in  the  higher 
orders,  are  separated  by  a  distinct  interval,  during  which  the 
germ-band  undergoes  a  considerable  development.  But  during 
this  interval,  the  diapause,  no  change  is  noticeable  in  the 
envelopes  themselves  beyond  a  thinning  of  the  amnion  with 
the  increased  growth  of  the  embryo.  The  elimination  of  the 
envelopes  is  preceded  by  katatrepsis  just  as  their  formation 
was  preceded  or  accompanied  by  anatrepsis.  This  elimination 
is  immediately  followed  by  the  completion  of  the  dorsal  body- 
wall  and  may  take  place  in  a  variety  of  ways.  Korschelt  and 
H eider  (’92)  distinguish  the  following  types  in  this  process: 

1.  The  amnion  and  serosa  become  continuous  and,  after  the 
eversion  of  the  embryo,  are  drawn  back  over  the  yolk  to  form 
a  single  layer  of  cells.  As  the  dorsad  growth  of  the  body-walls 
proceeds,  both  envelopes  are  drawn  together  and  pushed  into 
the  yolk  to  form  a  sack  or  longitudinal  tube  which  is  ultimately 
enclosed  by  the  walls  of  the  mesenteron  and  absorbed.  To 
this  type  belong  the  Odonata,  Rhynchota,  some  Orthoptera 
{Blatta,  QEcanthns,  Gryllotalpd)  and  some  Coleoptera  {Hy- 
drophihis) . 

2.  The  serosa  is  shed  from  the  yolk  and  the  amnion  alone 
contracts  on  the  dorsal  surface  preparatory  to  being  drawn  into 
the  yolk  and  absorbed.  (Certain  Coleoptera,  e.g.  Doryphoral) 

3.  The  serosa  alone  is  agglomerated  and  drawn  into  the 
dorsal  yolk,  the  amnion  being  cast  off.  (Certain  Diptera 
\ChironomiLS~\  and  Trichoptera.) 

4.  Both  envelopes  are  shed.  (Lepidoptera  and  certain 
Hymenoptera.) 

In  Xiphidium  we  may  perhaps  recognize  a  fifth  type,  in 
which  as  in  the  fourth,  both  amnion  and  serosa  are  shed.  But 
while  the  serosa  is  in  great  part  shed  as  a  simple  membrane, 
the  indusium  which  is  a  modified  portion  of  the  serosa,  together 
with  the  amnion  is  drawn  together  in  a  mass  and  cut  off  from 
the  embryo.  It  is  more  than  probable  that  other  types  of 
envelope  elimination  will  be  discovered  when  more  forms  have 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


been  studied.  Mtisca  may  perhaps  be  regarded  as  representing 
a  distinct  type,  since  in  this  highly  modified  form  the  rudi- 
mental  amnion  and  the  serosa  are  neither  shed  nor  agglomer¬ 
ated  and  engulfed  in  the  yolk,  but  are  supposed  to  form  the 
definitive  body- wall.  (Kowalewsky,  ’86;  Graber,  ’89.) 

It  is  clear  that  the  revolution  of  the  insect  embryo  includes 
three  distinct  processes:  first,  the  eversion  and  katatrepsis  of 
the  germ-band;  second,  the  formation  of  the  dorsal  walls;  and 
third,  the  elimination  of  the  envelopes.  The  mechanical  cause 
of  eversion  and  katatrepsis  is  probably  a  contraction  on  the  part 
of  the  envelopes  after  their  fusion  and  rupture  over  the  ventral 
surface  of  the  embryo.  After  the  embryo  is  everted  from  the 
amniotic  cavity,  or  exposed  after  the  rupture  of  the  amnion  and 
serosa,  these  envelopes  temporarily  form  the  dorsal  covering  of 
the  yolk.  Do  they  ever  form  the  definitive  dorsal  body-wall } 
For  both  envelopes  this  is  claimed  to  be  the  case  only  in  Mtisca. 
In  all  other  insects  the  serosa,  at  least,  takes  no  part  in  forming 
the  permanent  body-wall,  as  it  is  either  shed  or  engulfed  in  the 
yolk.  The  question  is,  therefore,  restricted  to  the  fate  of  the 
amnion.  In  many  insects  (Lepidoptera,  Hymenoptera  Phyto- 
phaga,  some  Diptera  and  Coleoptera),  it  has  been  shown  that 
the  amnion  takes  no  part  in  the  formation  of  the  definitive  body- 
wall,  although  a  decision  on  this  point  is  rendered  difficult  by 
the  fact  that  no  hard  and  fast  line  can  be  drawn  between  the 
ectoderm  of  the  germ-band  and  the  cells  of  the  amnion.  In 
other  insects  the  decision  is  even  more  difficult.  Still,  I  may 
say  that  I  have  seen  nothing  in  the  insects  I  have  studied, 
to  convince  me  that  the  amnion  is  converted  into  a  portion  of 
the  permanent  body-wall.  Even  in  Musca  it  seems  probable 
that  the  amnion  and  serosa  only  temporarily  function  as  the 
body-wall,  and  that  their  cells  are  ultimately  replaced  by  true 
ectodermal  elements  from  the  germ-band.  In  Blatta  and 
Xiphiditim  I  have  seen  appearances  which  lead  me  to  believe 
that  at  least  a  part  of  the  amnion  may  be  eliminated  by  such  a 
process  of  cell-substitution.  I  incline,  therefore,  to  the  views  of 
Korschelt  and  H eider  (’92),  who  hold  that  the  envelopes  are 
probably  completely  eliminated,  and  that  the  entire  body-wall 
is  derived  from  the  ectoderm  of  the  germ-band. 


8o 


WHEELER. 


[VOL.  VIII. 


If  this  be  the  correct  view,  it  follows  that  the  dorsal  body- 
wall  is  formed  in  essentially  the  same  manner  in  all  insects  — ■ 
by  a  growth  and  meeting  of  the  germ-band  edges.  This  pro¬ 
cess  is,  therefore,  remarkably  simple  and  uniform  compared 
with  the  processes  whereby  the  envelopes  are  eliminated.  The 
great  variability  in  the  latter  case  has  been  dwelt  on  by  Graber 
(’88)  in  a  paper  devoted  to  dorsal-wall  formation  in  the  Insecta. 
After  reviewing  all  the  literature  on  the  subject  and  contrib¬ 
uting  many  new  facts,  he  proceeds  to  base  a  classification  of 
the  insects  hitherto  studied,  on  the  “Keimhiillenzustande.” 
He  finds  some  fault  with  the  current  classification  on  the 
ground  that  insects  which  systematists  regard  as  closely  re¬ 
lated  often  present  great  differences  in  their  respective  methods 
of  dorsal-wall  formation,  whereas  remotely  related  insects  often 
agree  very  closely  in  this  respect.  Thus  Lina  and  Hydro- 
philus  differ  more  than  HydropJiilus  and  CEcantJms  in  the 
processes  whereby  the  dorsal-wall  is  formed.  In  considering 
Graber’ s  views,  I  may  pass  over  the  awkward  and  kakophonous 
nomenclature  which  he  has  introduced,  to  what  I  regard  as  his 
main  error,  viz.  the  superficial  analysis  of  his  subject.  Graber’ s 
term  Keimhiillenzustande,”  I  take  it,  includes  the  formation 
of  the  envelopes  as  well  as  their  condition  preceding  and 
during  their  elimination.  Now  I  have  attempted  to  show  that 
there  is  nothing  in  the  formation  of  the  envelopes  nor  in  the 
concomitant  anatrepsis  of  the  germ-band  in  the  different  insect 
orders  to  conflict  with  the  current  classification.  Nor  is  there 
anything  in  the  closure  of  the  dorsal-wall  in  different  groups 
—  restricting  this  term  to  the  confluence  of  the  pleural  edges 
of  the  germ-band  —  to  support  Graber’s  conclusion.  His  state¬ 
ment  must  therefore  be  restricted  to  the  elimination  processes. 
That  these  are  highly  variable  must  be  admitted,  but  they  are 
probably  of  very  little  taxonomic  value,  as  Graber  would  prob¬ 
ably  have  observed,  had  he  attempted  to  account  for  the  wide 
differences  in  allied  forms  and  the  agreement  of  remotely  re¬ 
lated  species.  It  is  my  ophiion  that  this  high  degree  of  varia¬ 
bility  in  the  elimination  process  is  to  be  traced  to  the  same 
causes  as  the  variability  of  the  indusium,  viz.,  the  rudimental 
character  of  the  envelopes.  Up  to  the  close  of  the  diapause 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  8 1 


the  envelopes  vSubserve  a  distinct  function,  but  as  soon  as  the 
germ-band  has  invested  the  yolk  with  its  own  ectoderm,  they 
have  become  functionless,  or  rudimental.  Long  before  this 
time,  in  fact  ever  since  their  completion,  the  envelopes  show 
no  traces  of  cell-division.  Moreover,  their  involution  into  the 
yolk  or  complete  shedding  shows  conclusively  that  their  mor¬ 
phological  value  is  at  this  time  reduced  to  7iiL  Whether  both 
envelopes  are  shed  instead  of  being  drawn  into  the  yolk,  or 
whether  one  is  shed  and  the  other  drawn  into  the  yolk,  may 
depend  to  some  extent  on  the  ease  with  which  the  pleural 
folds  can  close  without  their  temporary  assistance.  But  which 
of  these  processes  shall  occur  in  a  given  insect  is  probably  a 
matter  of  no  vital  importance  to  the  embryo,  and  has  prob¬ 
ably  played  no  role  in  the  struggle  for  existence.  The  involu¬ 
tion  of  the  envelopes,  it  is  true,  may  add  assimilable  matter  to 
the  embryo,  but  enough  energy  to  counterbalance  this  addition 
is  probably  consumed  in  metabolizing  the  dead  cells.  Hence 
the  adoption  of  this  process  may  be  of  no  greater  advantage  to 
the  embryo  than  the  complete  sloughing  of  the  useless  en¬ 
velopes. 

The  insect  envelopes,  therefore,  present  only  another  case  of 
an  organ  which  has  become  specialized  for  a  particular  function 
at  the  expense  of  its  formative  power.  This  same  phenomenon 
recurs  in  insect  ontogeny.  During  cleavage  certain  cells  are 
segregated  for  the  express  function  of  yolk-metabolization  (vi- 
tellophags),  while  the  remaining  cells  go  to  form  the  blastoderm. 
Later  the  cells  of  the  blastoderm  separate  into  those  of  the 
germ-band  proper  and  those  of  the  specialized  envelopes.  Still 
later,  if  the  insect  be  metabolic,  another  splitting  occurs,  a  por¬ 
tion  of  the  hypodermis  being  set  aside  in  the  form  of  the  imag- 
inal  disks  to  supplant  the  specialized  primitive  larval  hypoder¬ 
mis.  The  formative  material  of  the  insect,  like  that  of  other 
organisms,  thus  undergoes  a  successive  splitting  into  a  special¬ 
ized  and  a  comparatively  non-specialized  portion.  The  former, 
being  incapable  of  metamorphosis,  is  cast  off  or  broken  down, 
while  the  latter  persists  until  a  new  segregation  takes  place. 
The  analogy  of  this  process  to  that  occurring  in  rhizomatous 
plants,  Polyzoa,  etc.,  need  not  be  pointed  out  in  detail. 


82 


WHEELER. 


[VOL.  VIII. 


V.  Neurogenesis  in  the  Insecta. 

I .  The  Nerve-cord. 

The  first  traces  of  the  central  nervous  system  of  Xiphidium 
make  their  appearance  at  a  very  early  stage,  before  the  blasto¬ 
pore  is  closed  and  while  the  envelopes  are  still  incomplete.  In 
this  stage  (Fig.  2)  surface  preparations  made  according  to  the 
methods  given  in  the  latter  part  of  this  paper,  show  a  number 
of  pale  spots  scattered  over  the  procephalic  lobes.  They  fre¬ 
quently  occur  also  in  the  maxillary  region,  and,  were  it  possible 
to  remove  the  amnio-serosal  fold  without  injuring  the  surface  of 
the  germ-band,  would  probably  be  found  to  extend  still  further 
caudad.  The  meaning  of  these  spots  is  apparent  when  sections 
of  embryos  in  Stages  B-D  are  examined.  In  a  transverse  sec¬ 
tion  (Fig.  25)  through  the  middle  of  the  abdomen  of  an  embryo 
in  Stage  D,  the  ectoderm,  which  bulges  out  somewhat  on  either 
side  of  the  median  line,  is  seen  to  consist  of  two  kinds  of  ele¬ 
ments.  First,  there  are  a  few  large,  clear,  polygonal  cells  with 
spherical  nuclei  (nb^,  lying  in  the  deeper  portion  of  the  layer; 
and  second,  a  much  greater  number  of  small  and  more  deeply 
stainable  cells  (^^.),  differing  in  no  essential  respect  from  the 
cells  forming  the  remainder  of  the  ectodermal  layer,  such  as 
the  appendages.  The  latter  cells  have  smaller,  oval  or  cuneate 
nuclei,  which  appear  to  contain  more  chromatin  than  the  large 
inner  cells.  While  the  small  cells  form  a  continuous  layer,  the 
large  elements  make  their  appearance  singly  or  in  small  clusters, 
as  seen  in  the  figure.  It  is  these  pale  clusters  underlying  the 
darker  cells  which  produce  the  pale  spots  seen  from  the  surface. 

The  large  pale  cells  may  be  called  neuroblasts  —  since  it  is 
they  that  give  rise  to  the  purely  nervous  elements  of  the  cord.^ 

I  The  term  “neuroblast”  was  originally  used  by  Whitman  (’78  and  ’87)  to 
designate'  the  two  offspring  of  the  large  posterior  macromere  of  the  Clepsine  egg, 
which  give  rise  by  a  process  of  budding  to  two  rows  of  cells  —  the  “  neural  rows.” 
From  these  rows  the  nerve  cord  arises.  His  (’89)  subsequently  employed  the 
same  term  “  neuroblast  ”  to  designate  such  of  the  offspring  of  the  “  Keimzellen  ” 
as  give  rise  directly  by  differentiation  and  not  by  further  divisions  to  the  ganglion- 
cells,  or,  to  use  Waldeyer’s  term,  to  the  neurons  of  the  vertebrate  central  nervous 
system.  More  properly  the  term  would  have  been  applied  to  the  “  Keimzellen  ” 
themselves,  and  by  mistake  it  has  been  thus  used  by  at  least  one  recent  writer 
(C.  L.  Herrick,  ’92,  p.  430,  Fig.  10).  Haeckel  (Anthropogenie,  4th  ed.  p.  268^ 
’91)  uses  ‘  neuroblast  ’  in  the  sense  of  ectoderm  in  general. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


The  remaining  cells  which  cover  the  neuroblasts  and  extend 
down  between  them  in  the  median  line,  give  rise  to  purely 
integumental  structures  and  may  therefore  be  called  dermato- 
blasts.  The  two  thickenings  of  the  ectoderm  are  to  become 
the  lateral  cords  (Seitenstrange).  They  extend  from  the 
anterior  edge  of  the  eleventh  abdominal  segment,  just  in  front 
of  the  anus,  to  the  mouth,  where  they  diverge  and  pass  without 
interruption  into  the  brain.  The  groove  which  separates  the 
lateral  cords  and  which  is  very  faint  in  Fig.  25,  is  the  neural 
furrow  (Primitivrinne).  It  appears  soon  after  the  closure  of 
the  blastopore  and  takes  the  place  of  this  depression.  It  is 
deepest  anteriorly. 

All  the  neural  structures  develop  in  an  anteroposterior  direc¬ 
tion,  beginning  with  the  brain;  hence  different  stages  in  the 
development  of  the  lateral  cords  may  be  studied  in  the  same 
embryo.  Fig.  26  shows  a  section  passing  through  the  first 
abdominal  segment  of  the  embryo  from  which  the  section 
in  Fig.  25  was  taken.  Here  we  see  a  distinct  advance  in 
structure.  The  neural  furrow  is  more  clearly  marked 

and  the  neuroblasts  four  in  either  lateral  cord,  have 

arranged  themselves  side  by  side  in  a  regular  layer  in  the 
deepest  portion  of  the  ectoderm.  Over  them  the  dermatoblasts 
{db})  also  form  a  single  regular  layer,  while  the  cells  lying  in 
the  median  line  on  either  side  of  the  neural  furrow  have  grown 
more  elongate.  Sections  further  forward  show  essentially  the 
same  conditions  —  the  neuroblasts  which  were  at  first  differ¬ 
entiated  as  small  clusters  or  as  isolated  cells,  have  arranged 
themselves  throughout  the  anterior  portion  of  the  embryo  as 
an  even  layer  entad  to  the  dermatoblasts. 

The  further  changes  in  the  development  of  the  nerve-cord, 
are  brought  about  —  first,  by  a  proliferation  of  the  neuroblasts  ; 
second,  by  a  proliferation  of  the  dermatoblasts  and  a  deepening 
of  the  neural  furrow;  third,  by  the  development  of  the  median 
cord;  fourth,  by  the  formation  of  the  connectives  and  com¬ 
missures,  and  fifth,  by  the  development  of  the  neurilemmata. 
These  changes  which  occur  simultaneously  may  be  described 
singly  for  the  sake  of  convenience. 

As  cross-sections  show,  the  neuroblasts  are  arranged  in  from 


84 


WHEELER, 


[VOL.  VIII. 


3-5  longitudinal  rows  in  either  lateral  cord.  In  surface  view 
these  rows  may  often  be  followed  through  one  or  two  seg¬ 
ments  as  continuous  strings  of  cells.  I  assume  that  there 
were  originally  four  of  these  rows,  but  that  owing  to  the 
pressure  exerted  by  the  developing  appendages  on  the  lateral 
edges  of  the  cords  and  to  a  more  rapid  growth  of  the  neuro¬ 
blasts  than  of  the  germ-band,  the  primitive  regular  arrange¬ 
ment  has  been  considerably  obscured.  The  neuroblasts  are 
polygonal  in  outline  from  mutual  pressure.  When  they  divide, 
as  they  very  soon  do,  their  spindle  axes  are  directed  at  right- 
angles  to  the  surface  of  the  body.  As  soon  as  one  cell  has 
been  given  off,  the  nucleus  rests  for  a  short  time  and  then 
again  divides  in  the  same  direction.  This  process  continuing, 
a  column  of  cells  is  budded  off  from  each  neuroblast  and  stands 
at  right  angles  to  the  surface  of  the  germ-band.  The  divisions 
do  not  take  place  simultaneously  in  all  the  cells  although  cor¬ 
responding  neuroblasts  in  either  cord  will  frequently  be  found 
in  the  same  phase  of  caryokinesis,  especially  in  the  earlier 
stages  of  their  proliferation.  A  section  (Fig.  27)  through  the 
first  maxillary  segment  of  an  embryo  in  Stage  F  shows  that 
each  of  the  eight  neuroblasts  has  produced  a  row  of  daughter- 
cells.  The  large  succulent  mother-cells  are  evenly  rounded  on 
their  outer  surfaces  which  are  overlaid  by  the  dermatoblasts. 
Their  inner  faces  are  fiat  or  concave  and  in  every  case  closely 
applied  to  the  latest  daughter-cell.  The  nuclei  of  the  mother- 
cells  are  spheroidal  and  take  no  deeper  stain  than  the  pale  suc¬ 
culent  cytoplasm  which  surrounds  them.  The  neuroblasts  are 
in  all  essential  respects  typical  proliferating  cells  like  the  ter¬ 
minal  cells  in  plant-shoots  and  the  teloblasts  of  annelids.  The 
daughter-cells  are  at  first  characterized  by  their  small 

size,  cuneate  outline  and  deep  stain.  Their  nuclei  are  con¬ 
siderably  flattened,  probably  from  mutual  pressure.  These 
characters  are  retained  by  the  daughter-cells  till  they  have  been 
pushed  some  distance  from  the  neuroblast  by  later  offspring, 
when  they  become  larger  and  considerably  paler  and  assume 
the  appearance  of  definitive  ganglion  cells  {g-). 

Turning  now  to  a  somewhat  older  embryo  (Stage  G,  Fig.  28) 
we  see  that  the  columns  of  daughter-cells  have  greatly  increased 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  85 


in  length,  while  the  neuroblasts  remain  to  all  appearances  un¬ 
altered.  The  increase  in  the  number  of  daughter-cells  is  so 
great  that  they  are  forced  to  arrange  themselves  in  several 
rows.  In  the  figure  this  is  best  shown  in  the  progeny  of  the 
innermost  neuroblasts,  and  the  tapering  columns  there  formed 
may  be  regarded  as  typical. 

In  my  preliminary  note  (’9i®)  I  held  that  the  daughter-cells 
themselves  divide  to  form  the  multiple  rows  in  each  pillar.  I  in¬ 
cline  to  think  that  I  was  mistaken  on  this  point.  The  daughter- 
cells  probably  never  divide  but  are  directly  converted  into 
ganglion  cells.  All  reproductive  powers  seem  to  be  confined  to 
the  neuroblasts.  Some  of  the  nuclei  of  the  daughter-cells  exhibit 
peculiar  chromatic  structures  which  I  may  have  mistaken  for 
caryokinetic  figures  ;  this  being  an  easy  error  to  make  in  the 
case  of  small  cells  killed  by  means  of  heat,  since  the  achromatic 
portions  of  the  spindles  are  obliterated  by  this  method. 

The  last  stages  in  the  proliferation  of  the  neuroblasts  are 
shown  in  Fig.  31,  which  is  taken  from  an  advanced  embryo 
(Stage  J).  The  columnar  arrangement  is  no  longer  visible 
since  the  individual  cells  are  now  converted  into  the  de¬ 
finitive  ganglionic  elements.  On  the  outer  periphery  of  the 
ganglia,  however,  neuroblasts  are  still  to  be  found,  and  ex¬ 
tending  from  them  short  series  of  small  flattened  cells 
their  latest  progeny,  still  distinguishable  from  the  ganglion 
cells  by  their  deeper  stain.  It  will  be  noted  that  these  cells, 
which  like  their  precursors  will  become  ganglion  cells,  are  no 
longer  budded  off  at  right  angles  to  the  surface  of  the  nerve- 
cord  but  parallel  to  it,  a  condition  undoubtedly  due  to  a  lack 
of  space.  Finally  the  neuroblasts  stop  proliferating  and  shrink 
to  the  size  of  their  progeny.  Their  chromatin  then  shows 
signs  of  senility.  Beyond  this  point  I  have  been  unable  to 
trace  them  satisfactorily.  They  are  probably  broken  down  and 
absorbed  by  the  growing  ganglion  cells.  Some  of  them  may 
persist  as  ganglion  cells  of  a  particular  character  and  function, 
though  I  deem  this  improbable. 

The  dermatoblasts  play  an  important  part  in  the  develop¬ 
ment  of  the  ventral  nerve-cord,  as  will  be  seen  by  returning  to 
the  younger  stages.  We  left  these  cells  as  a  layer  covering 


86 


WHEELER. 


[VOL.  VI 11. 


the  neuroblasts  and  continuous  laterally  with  the  general  ecto¬ 
derm.  In  the  median  line  they  extend  to  the  deepest  portion 
of  that  germ-layer  in  the  form  of  a  few  compressed  cells  (Fig. 
26,  db).  These  compressed  cells  form  the  walls  and  bottom  of 
the  neural  furrow.  The  proliferation  of  the  neuroblasts  has 
caused  the  lateral  cords  to  bulge  out  enormously  (Fig.  27),  so 
that  the  dermatoblastic  layer  becomes  stretched  and  attenuated. 
Such  divisions  as  occur  in  the  cells  of  this  layer  seem  to  be 
confined  to  the  outer  surface  and  do  not  extend  into  the  furrow. 
The  spindle  axes  lie  parallel  to  the  surface,  as  shown  at  nh. 
The  bulging  of  the  lateral  cords  naturally  brings  about  a  deep¬ 
ening  of  the  neural  furrow,  since  the  cells  at  its  bottom  have  a 
fixed  attachment.  At  this  point  in  Fig.  27  there  is  seen  a 
triangular  cell-mass,  capped  by  a  single  large  element  (innb.),  a 
true  neuroblast  which  resembles  in  nearly  all  respects  the 
neuroblasts  of  the  lateral  cords.  Its  more  pyramidal  outline  is 
obviously  the  result  of  its  position  between  the  converging 
walls  of  the  furrow.  To  the  same  mechanical  cause  is  due  the 
shape  of  the  cell-mass,  which  consists  of  the  heaped  up 
daughter-cells  of  the  neuroblast.  Inasmuch  as  the  proliferating 
cell  occurs  in  the  median  line,  and  together  with  its  offspring 
and  the  dermatoblastic  cells  of  the  median  furrow,  is  equivalent 
to  the  “  Mittelstrang  ”  of  authors,  I  shall  call  it  the  median- 
cord  neuroblast.  Its  exact  origin  I  have  not  been  able  to 
determine.  To  judge  from  the  number  of  cells  which  it  has 
given  off  it  must  have  begun  to  proliferate  at  about  the  same 
time  as  the  lateral-cord  neuroblasts.  There  can  be  no  doubt, 
it  seems  to  me,  that  it  originated  as  a  polygonal  ectoderm  cell 
like  the  lateral  cells  seen  in  Figs.  25  and  26,  but  whether  it 
was  originally  median  in  position  or  arose  unilaterally  I  am 
unable  to  decide.  The  pale  surface  spots  of  embryos  in  Stage 
B  show  that  neuroblasts  are  arising  at  a  time  when  the  blasto¬ 
pore  occupies  the  position  of  the  neural  furrow  and  hence,  if 
the  median  cells  are  median  in  position  from  the  first,  they 
must  arise  somewhat  later  than  their  sister  neuroblasts. 

There  is  one  important  difference  in  the  arrangement  of  the 
mother-cells  of  the  lateral  and  median  cord.  Whereas  the  for- 
mer,  as  has  been  stated,  form  continuous  though  irregular  rows 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY,  87 


from  mouth  to  anus,  the  latter  constitute  an  interrupted  series 
between  the  same  two  points.  They  are  single,  isolated  cells, 
which  occur  only  intersegmentally.  That  such  is  their  distribu¬ 
tion  may  be  distinctly  seen  in  frontal  sections  like  the  one  rep¬ 
resented  in  Fig.  30.  This  section  passes  through  the  first 
to  fifth  abdominal  segments  at  the  level  of  the  median  cord 
neuroblasts  {innb),  which  are  seen  to  lie  distinctly  between 
the  segments,  where  the  walls  of  the  neural  furrow  dilate 
at  intervals  for  their  accommodation.  At  first  the  daughter- 
cells  are  given  off  in  the  same  direction  as  those  of  the  lateral 
cords,  but  soon  the  triangular  space  to  which  they  are  confined 
will  no  longer  contain  the  older  cells  of  the  series  and  these 
are  pushed  along  the  floor  of  the  neural  furrow.  This  produces 
an  angular  flexure  in  the  cell-column,  but  later  the  whole  mass, 
including  the  neuroblast,  assumes  a  horizontal  position.  This 
change  in  the  position  of  the  median  cell-mass  is  seen  to  have 
taken  place  in  the  median  sagittal  section  from  an  embryo  in 
Stage  G  (Fig.  29).  The  neuroblast  i^nmb)  is  in  each  segment 
directed  caudad,  while  the  mass  of  small  and  deeply  stainable 
daughter-cells  {mg)  is  wedged  in  under  the  commissures.  The 
section  passes  through  the  sub-oesophageal  ganglion,  which 
consists  of  the  fused  ganglia  of  the  mandibular  and  both  maxil¬ 
lary  segments  iind.  g;  mx.  g^ ;  mx.  g-),  and  through  the  pro-  and 
mesothoracic  ganglia  Transverse  furrows  (f.  ; 

i.g^)j  which  I  shall  consider  later,  separate  the  unfused  ganglia 
from  one  another,  and  as  the  median  cord  cells  lie  in  front  of 
these  furrows,  they  must  be  regarded  as  belonging  not  to  the 
intersegmental  region  of  the  ectoderm,  but  to  the  posterior 
portions  of  the  separate  ganglia.  Each  ganglion  possesses  a 
median  cord  neuroblast,  so  that,  beginning  with  the  mandibular, 
which  is  the  first  ganglion  in  the  nerve-cord  proper,  and  ending 
with  the  tenth  abdominal,  there  are  in  all  sixteen  median 
mother-cells.  Each  of  these,  after  producing  its  quota  of 
ganglionic  elements,  deteriorates  in  the  same  way  as  the 
mother-cells  of  the  lateral  cords. 

The  development  of  the  Punktsubstanz  may  be  readily  fol¬ 
lowed  in  XipJiidium.  It  arises  in  each  ganglion  as  two  separate 
masses.  Each  of  the  daughter-cells  of  the  lateral  neuroblasts 


88 


WHEELER. 


[VOL.  VIII. 


sends  out  a  cytoplasmic  process  which  soon  ramifies.  The  mass 
of  fibres  thus  formed  increases  in  size  very  probably  by  the  addi¬ 
tion  of  further  ramifications  till  the  Punktsubstanz  is  definitely 
established  as  a  scarcely  stainable  body,  lying  on  either  side  of 
the  median  line  in  the  deepest  portion  of  the  lateral  cord  (Fig. 
27, /.j.)  In  its  earliest  stages  the  formation  of  the  substance 
is  easily  followed,  but  very  soon  the  felted  fibres  become  too 
dense  for  analysis  by  ordinary  methods  of  investigation.  It  is 
only  after  a  distinct  mass  is  formed  in  either  half  of  a  ganglion 
that  the  longitudinal  commissures,  or  connectives  as  they  are 
best  called,  make  their  appearance,  and  unite  the  hitherto  iso¬ 
lated  centres  in  two  longitudinal  series.  Very  soon  the  trans¬ 
verse  commissures,  or  commissures  proper,  of  which  Xiphidiumy 
like  all  other  insects,  has  two  in  each  segment,  make  their 
appearance.  The  daughter-cells  of  the  median  cord  neuroblasts 
take  no  part  in  the  formation  of  the  anterior  commissure. 
Whether  they  contribute  fibres  to  the  posterior  commissure  or 
not,  I  must  for  the  present  leave  undecided.  I  have  seen  no 
evidence  in  the  median  cord  of  a  distinct  and  isolated  Punkt¬ 
substanz  centre,  such  as  is  described  and  figured  by  Graber  for 
some  Coleoptera  ((’90)  PI.  V,  Fig.  66).  I  deem  it  more  probable 
that  in  Xiphidium  the  commissures  arise  wholly  from  the 
Punktsubstanz  masses  of  the  lateral  cords.  Both  commissures 
are  distinctly  seen  in  cross  section  in  Fig.  29. 

The  connectives  and  commissures  incompletely  divide  the 
cellular  portion  of  each  ganglion  into  five  parts,  —  two  lying 
laterad  to  the  connectives  and  a  median  series  of  three  smaller 
portions  separated  from  one  another  by  the  two  commissures. 
The  former  may  be  called  lateral  gangliomeres,  and  the  three 
median  portions  respectively  the  anterior,  central,  and  poste¬ 
rior  gangliomere.^  Of  the  median  divisions  the  posterior  is 
distinctly  the  largest  from  the  first.  This  is  due  to  its  being 
formed  in  great  part  by  the  progeny  of  the  median  neuroblast, 
whereas  the  anterior  and  central  gangliomeres  consist  of  a 
comparatively  small  number  of  cells,  contributed  by  the  lateral 
neuroblasts. 

1  These  are  equivalent  to  Graber’s  laterale  Zellenlager,  vorderes,  centrales,  and 
hinteres  Medianlager. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  89 


It  is  not  till  after  the  commissures  and  connectives  are 
formed  that  the  inter-ganglionic  regions  become  clearly  marked 
out.  Throughout  the  early  stages,  in  fact  till  the  embryo 
reaches  the  ventral  surface  of  the  egg  (Stage  J),  the  ganglia 
are  as  long  as  their  respective  segments  and  are  separated 
from  one  another  only  by  the  intersegmental  constrictions. 
These  have  grown  very  deep  in  Stage  G,  especially  in  the 
thoracic  and  abdominal  regions.  In  the  median  line,  as  shown 
in  sagittal  section  (Fig.  29,  i.g^^  they  form  deep  tubular 

ingrowths  which  may  be  called  furcal  pits.  Since  these  pits 
are  median  in  position  they  are  to  be  regarded  as  differentiated 
interganglionic  portions  of  the  neural  furrow.  They  therefore 
belong  to  the  median  cord.  They  are  not  found  between  the 
mandibular  and  first  maxillary,  nor  between  this  and  the  second 
maxillary  ganglion,  and  are  also  wanting  between  the  eighth 
and  ninth,  and  ninth  and  tenth  abdominal  ganglia.  Evidently 
their  absence  in  these  cases  is  due  to  early  fusion  to  form  the 
infraoesophageal  and  last  abdominal  ganglion.  In  the  thoracic 
segments  the  furcal  pits  are  converted  into  chitinous  apode- 
matous  structures  which  give  attachment  to  some  of  the  leg- 
muscles.  It  is  interesting  to  note  that  in  the  abdomen  also 
furcal  pits  are  distinctly  developed  as  late  as  Stage  K.  Here, 
too,  they  serve  for  the  attachment  of  a  few  weak  muscle-like 
structures,  which  run  from  their  tips  to  points  in  the  adjacent 
abdominal  wall,  perhaps  corresponding  to  the  insertions  of  the 
rudimental  appendages.  Later  both  muscle-like  cords  and  ab¬ 
dominal  furcae  disappear,  — the  latter  by  a  very  simple  process. 
It  will  be  remembered  that  at  this  time  the  embryo  is  growing 
in  length  and  continually  covering  more  and  more  of  the  yolk. 
The  tail  end  is  practically  fixed  at  the  lower  pole  of  the  egg, 
while  the  head  slowly  moves  upwards.  The  body-wall  is  thus 
stretched  in  both  a  longitudinal  and  lateral  direction.  Hence 
the  intersegmental  constrictions,  so  deep  in  Stage  J,  must 
gradually  become  shallower,  and  the  furcal  pits,  which  are 
nothing  but  portions  of  these  constrictions,  are  drawn  out  from 
between  the  connectives  to  form  part  of  the  sternal  integu¬ 
ment.  The  stretching  not  only  draws  out  the  folds  in  the  em¬ 
bryonic  body-wall,  but  also  reduces  it  to  a  much  thinner  layer 


90 


WHEELER. 


[VOL.  VIII. 


of  cells.  The  length  of  the  individual  segments  is  thereby 
greatly  increased  and  the  nerve  cord,  which  is  firmly  attached 
in  the  infraoesophageal  region  and  more  loosely  in  the  terminal 
abdominal  segments,  is  compelled  to  lengthen.  The  separate 
ganglia,  besides  assuming  a  somewhat  fusiform  outline,  are 
scarcely  affected  by  this  traction,  whereas  the  connectives  are 
drawn  out  into  thin  threads  denuded  of  all  ganglionic  cells  and 
covered  only  by  the  neurilemma. 

The  presence  in  the  abdomen  of  temporary  furcal  pits  cor¬ 
responding  to  the  persistent  furcas  of  the  thorax  admits  of  an 
easy  explanation,  if  we  take  these  structures  to  be  correlated 
with  the  development  of  ambulatory  appendages.  The  tem¬ 
porary  abdominal  appendages  have  usually  been  regarded  as 
the  rudiments  of  once  functional  walking-legs,  and  they  are  still 
so  well  preserved  in  the  Orthoptera  that  it  need  not  surprise 
us  to  find  traces  of  correlated  structures  which  served  for  the 
attachment  of  some  of  their  muscles. 

The  progeny  of  the  median  neuroblast  together  with  the 
interganglionic  portion  of  the  neural  furrow  have  been  ac¬ 
counted  for;  the  former  becoming  the  posterior  gangliomere, 
the  latter  a  portion  of  the  sternal  integument  ;  but  I  have  not 
yet  accounted  for  the  remaining  portion  of  the  median  cord  — 
viz.  the  intraganglionic  walls  of  the  neural  furrow.  This 
portion  of  the  groove  is  crossed  by  the  two  commissures  and 
separates  those  portions  of  the  lateral  cord  which  will  ulti¬ 
mately  constitute  the  anterior  and  central  gangliomeres.  Its 
cells  are  of  an  epithelial  nature.  Those  of  the  opposite 
walls  of  the  furrow  become  applied  to  one  another  by  the 
swelling  of  the  lateral  cords.  The  lumen  is  thereby  oblit¬ 
erated  though  its  walls  are  still  continuous  on  the  outer 
surface  of  the  ganglion  with  the  integumental  ectoderm. 
The  two  lips  of  the  furrow  finally  fuse  and  the  ganglion 
together  with  the  portion  of  the  furrow  included  between 
its  two  halves  is  liberated  from  the  ectoderm.  It  is  these 
epithelial  walls  thus  set  free  from  the  integument  which 
appear  to  give  rise  to  the  outer  and  inner  neurilemmata. 
Both  these  neural  envelopes  are  ectodermal  ;  there  are  no 
traces  of  mesodermal  structures  taking  any  part  in  their  forma- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


91 


tion  and  it  seems  to  me  that  they  can  have  only  two  possible 
sources  —  they  either  arise  from  some  of  the  progeny  of  the 
neuroblasts  or  from  the  intraganglionic  portion  of  the  median 
cord.  I  deem  it  highly  improbable  that  they  should  arise  from 
the  former  source,  since  the  daughter-cells  of  the  neuroblasts 
have  every  appearance  of  being  early  specialized  as  ganglion 
cells.  Furthermore,  the  cells  of  the  neurilemmata  when  they 
definitely  appear,  closely  resemble  the  cells  of  the  neural  fur¬ 
row  both  in  size  and  in  the  great  avidity  with  which  they 
take  the  stain.  The  outer  neurilemma  covers  first  the  inner 
surface  of  the  ganglion  —  then  the  outer  or  neuroblastic  sur¬ 
face; —  the  thin  cellular  membrane  apparently  progressing 
ing  laterad  in  either  case  and  meeting  near  the  origin  of  the 
nerve-trunks.  The  inner  neurilemma,  which  envelops  the 
Punktsubstanz  is  completed  before  the  outer  envelope.  Histo¬ 
logically  both  envelopes  resemble  each  other  in  every  respect. 

The  fusions  of  ganglia  in  the  nerve-cord  take  place  gradually 
and  may  be  easily  followed  in  Xiphidiuni.  Several  stages  of 
these  fusions  are  represented  in  Fig.  VII,  A-D.  In  Fig.  A, 
the  nerve-cord  is  shown  much  as  it  appears  in  Stage  F.  The 
ganglia  form  an  unbroken  series  from  mouth  to  anus.  The 
connectives  are  very  short  and  not  as  yet  distinguishable  from 
the  surface.  Fig.  B,  is  taken  from  an  embryo  just  turning  the 
lower  pole  (Stage  H).  Here  the  mandibular  and  two  maxillary 
ganglia,  and  also  the  three  terminal  abdominal  ganglia  still 
remain  as  in  the  preceding  stage,  while  the  other  ganglia  are 
being  drawn  apart  by  the  stretching  of  the  embryo,  so  as  to 
show  their  short  connectives.  In  Fig.  C,  the  suboesophageal 
and  last  abdominal  ganglia  are  established  as  two  fused  masses. 
The  number  of  ganglia  comprising  each  of  these  masses  may 
still  be  easily  determined  by  counting  the  commissures.  It 
will  be  noticed  that  in  this  stage  the  first  abdominal  is  closely 
approximated  to  the  metathoracic  ganglion  and  that  the  second 
and  third  abdominal  also  lie  close  together.  Between  the 
other  ganglia  the  connectives  have  lengthened.  In  the  later 
stage  represented  in  Fig.  D,  the  connectives  are  still  longer; 
the  first  abdominal  ganglion  has  fused  with  the  metathoracic, 
and  the  second  and  third  abdominal  form  a  single  mass. 


92 


WHEELER. 


[VOL.  VIII. 


These  fusions  become  more  intimate  as  the  time  for  hatching 
approaches,  so  that  the  ventral  cord  finally  consists  of  only 
ten  ganglia  instead  of  sixteen,  the  original  number. 


A-D.  Diagrams  of  four  consecutive  stages  in  the  development  of  the  brain 
and  nerve-chain  of  the  Xiphidium  embryo.  I,  cephalic ;  II,  thoracic ;  III, 
abdominal  region  ;  st.,  stomodeeum  ;  an.,  anus  ;  e,  optic  plate  ;  pd{o.g),  first 
protocerebral  lobe,  or  optic  ganglion  ;  pc^,  pc^,  second  and  third  protocerebral 
lobes  ;  dc,  deutocerebrum  ;  tc.,  tritocerebrum  ;  i-i6,  the  sixteen  postoral  ganglia  ; 
po.c.,  postoral  commissure  ;  fp.,  furcal  pit  ;  ac.,  anterior ;  pc.,  posterior  ganglionic 
commissure  ;  ag.,  anterior  ;  pg.,  posterior  ;  eg.,  central ;  Ig.,  lateral  gangliomeres. 

The  description  of  the  ventral  nerve-cord  of  Xiphidmm  here 
given  applies  equally  well  to  the  other  Orthoptera  which  I  have 
studied  {Blatta  gennanica,  MelariopliLS  feimtr-riibritDt).  The 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY, 


93 


points  in  which  the  Blattid  and  Acridian  nerve-cord  differ  from 
that  of  the  Locustid  are  so  insignificant  that  I  need  not  burden 
the  reader  with  their  enumeration.  I  will  stop  to  mention  only 
two  peculiarities  in  Blatta.  Here  I  fail  to  detect  the  pale 
spots  in  the  “slipper”  stage  of  the  germ-band,  and  sections 
show  that  the  neuroblasts  do  not  differentiate  as  early  as 
they  do  in  Xiphidiiim.  They  are,  however,  readily  detected 
in  late  stages,  when  they  stand  out  with  even  greater  distinct¬ 
ness  than  in  the  Locustid.  The  median  cord  neuroblasts,  ' 
though  present  and  occupying  positions  corresponding  to  their 
homologues  in  Xiphidmm,  are  more  difficult  to  trace,  probably 
on  account  of  the  smaller  size  of  the  embryo. 

Neuroblasts,  or  cells  of  a  similar  character  have  been  de¬ 
scribed  and  figured  by  a  number  of  investigators  of  Arthropod 
development.  Perhaps  the  earliest  mention  of  these  cells  is 
to  be  found  in  Reichenbach’s  beautiful  Astacits  monograph 
(’86),  where  the  nerve-cord  is  described  as  consisting  in  an 
early  stage  of  two  kinds  of  cells  —  a  few  large  pale  elements 
arranged  in  a  single  layer  and  confined  to  the  periphery,  and 
a  much  greater  number  of  small  and  more  deeply  stainable 
cells  forming  the  bulk  of  the  ganglia.  The  developing  ganglia 
of  the  cray-fish  resemble  the  ganglia  of  the  Orthoptera  in  many 
particulars.  The  number  of  large  cells  in  the  lateral  cords  in 
Reichenbach’s  figures  (notably  his  figures  1 14-133)  is  3-6,  the 
average  being  4  or  5,  the  same  as  in  XipJiidiiim^  Blatta^  etc. 
Furthermore  the  ganglia  of  Astacus  show  a  foliated  arrange¬ 
ment  of.  the  smaller  cells,  which  is  not  unlike  the  condition 
seen  in  the  older  ganglia  of  the  Orthoptera.  Some  of  the 
figures  (188  and  189  for  example)  show  a  single  neuroblast¬ 
like  cell  surmounting  the  median  cord  cell-mass.  There  are, 
however,  two  points  in  Reichenbach’s  work,  which  throw  some 
doubt  on  the  homology  of  his  large  cells  with  the  neuroblasts 
of  the  Orthoptera.  First,  Reichenbach  neither  figures  nor 
deseribes  these  cells  as  dividing  to  form  ganglion  cells.  This 
negative  observation,  however,  loses  much  of  its  force  when 
we  consider  that  caryokinetic  figures  are  singularly  absent 
from  all  of  Reichenbach’s  figures,  excepting  his  surface  views 
of  young  embryos,  and  when  we  recall  the  fact  that  amitotic 


94 


WHEELER. 


[VOL.  VIII. 


division  is  a  very  general  phenomenon  in  the  Crustacea  (ac¬ 
cording  to  Carnoy,  ’85).  A  more  serious  objection  to  the 
homology  under  consideration  is  Reichenbach’s  statement  that 
the  large  cells  ‘gehen  schliesslich  in  die  von  Leydig,  Dietl, 
Krieger  und  anderen  beschriebenen,  grossen  Ganglienzellen 
iiber.’  This,  too,  is  an  objection  only  if  the  neuroblasts  really 
degenerate,  a  point  on  which  I  am  still  doubtful. 

Nusbaum  found  huge  succulent  cells  in  the  young  nerve 
cord  of  the  embryo  My  sis  (’87).  He  compares  them  with  the 
large  cells  of  Reichenbach  and  believes  that  they  have  a  similar 
fate.  Similar  cells  were  also  observed  in  the  brain  of  Onisctis 
^niLrariiis  ‘‘pendant  les  stades  relativement  jeunes.”  On  one 
point  only  does  he  add  to  Reichenbach’s  observations:  he 
depicts  (Fig.  78)  a  caryokinetic  figure,  which  from  its  size 
and  position  must  be  referred  to  one  of  the  large  cells.  Its 
spindle  axis  is  directed  at  right  angles  to  the  surface  of  the 
body.  This  observation,  small  and  incidental  as  it  is,  would 
tend  to  show  that  the  large  cells  proliferate  as  in  the  Orthop- 
tera.  I  am  inclined  to  think  that  a  renewed  study  of  the 
Crustacean  nerve  cord  will  show  that  the  ganglion  cells  are 
budded  forth  from  the  large  cells  and  that  these  are  equivalent 
to  the  insect  neuroblasts. 

Korotneff  (’85)  was  the  first  to  find  gangliogenic  cells  in 
the  Insecta.  At  p.  589  in  his  description  of  the  Gryllotalpa 
embryo  he  says:  “  Einige  der  Ektodermzellen,  welche  die 
Nervenauftreibung  bedecken,  fangen  an  zu  w'-achsen,  ihre 
Kerne  vergrossern  sich  bedeutend  und  zeigen  dabei  eine  karyo- 
kinetische  Figur.  Grosstentheils  sind  diese  Zellen  (Ganglien) 
so  angeordnet,  dass  einer  einfachen  platten  Ektodermzelle 
eine  wachsende  Neuroektodermzelle  folgt.  Hat  sie  eine  be- 
stimmte  Grosse  erreicht,  so  sinkt  jede  wachsende  Zelle  in  die 
Tiefe  des  Ektoderms  und  wird  von  den  benachbarten,  unver- 
anderten  Zellen  bedeckt.  Jede  Ganglienzelle  theilt  sich  dabei, 
eine  ganze  Folge  von  neuentstandenen  Zellen  bildend,  nur  an 
der  Fig.  60  ist  leicht  zu  unterscheiden,  welche  Gruppe  von 
Zellen  der  oben  gelegenen  Ganglienzelle  entspricht.  Durch 
eine  solche  Vermehrung  von  Zellen  wird  der  Nervenstrang 
mehr  und  mehr  in  die  Hohe  getrieben.”  The  Fig.  60  referred 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


95 


to  in  this  description  is  taken  from  a  stage  corresponding  to 
my  Fig.  28.  Both  in  this  figure  and  in  Fig.  61  he  represents 
four  neuroblasts  in  one  of  the  lateral  cords.  Korotneff  seems 
not  to  have  seen  the  early  stages  of  proliferation. 

In  the  developing  nerve-cord  of  Doryphora  I  observed  (’89,  p. 
366)  that  “the  outer  layer  of  cells  continuous  with  the  hypo- 
dermis  stands  off  somewhat  from  the  ganglionic  thickenings, 
leaving  a  space  which  is  in  early  stages  occupied  by  several 
large,  clear,  oval  cells  which  divide  rapidly  by  caryokinesis,  and 
might  be  called  ganglioblastSy  as  the  products  of  their  divisions 
reinforce  the  mass  of  ganglion  cells.”  In  my  figures  the  polar- 
axes  of  the  neuroblast  spindles  lie  parallel  to  the  surface  of  the 
ganglia.  Re-examination  of  my  preparations  has  convinced 
me  that  this  observation  is  essentially  correct.  I  find  also  that 
the  newly-formed  daughter-cells  of  the  neuroblasts  occasionally 
divide  caryokinetically  and  thus  give  rise  to  further  generations 
of  daughter-cells.  The  daughter-cells  are  not  budded  forth  in 
regular  rows,  but  very  irregularly.  I  am  not  sure  that  I  can 
distinguish  the  median  cord  neuroblasts  in  Doryphora,  though 
I  believe  that  I  have  detected  homologous  structures.  In  my 
figure  72  I  represented  circular  intersegmental  patches  in  the 
median  line  between  the  lateral  cords.  Closer  examination 
shows  these  to  be  clusters  of  cells  of  the  same  appearance  and 
dimensions  as  the  lateral-cord  neuroblasts.  They  are  very 
clearly  brought  out  by  Graber  in  his  figures  of  Hydrophilus 
(’89,  Figs.  40,  41,  and  43,  PI.  Ill)  and  are  described  as  taking 
part  in*  the  formation  of  the  posterior  gangliomere  of  each 
ganglion.  I  doubt  whether  the  large  cells  constituting  the 
posterior  gangliomere  of  Periplaneta  in  Miall  and  Denny’s 
Fig.  43  (’86)  to  which  Graber  refers,  are  to  be  regarded  as  the 
equivalents  of  the  median-cord  clusters  in  Doryphora  and 
Hydrophilus.  Peripla^teta  very  probably  has  in  each  segment 
only  one  median-cord  neuroblast,  which  atrophies  before  the 
close  of  embryonic  life,  and  the  large  cells  in  Miall  and 
Denny’s  figure  probably  arise  from  the  daughter-cells  and  are 
therefore  merely  large  ganglionic  elements. 

Graber  figures  and  describes  (’89,  p.  47,  PI.  X,  Fig.  130)  a 
cross-section  through  an  abdominal  ganglion  of  a  Melolontha 


96 


WHEELER. 


[VOL.  VIII. 


embryo  in  which  he  finds  a  neuroblast  in  either  lateral  cord 
and  three  symmetrically  arranged  cells  of  the  same  character 
in  the  median  cord.  Similiar  median  cells  were  seen  in  LiLcilia. 
He  refers  to  Korotneff’s  observations  on  Gryllotalpa  and  states 
that  he  has  found  the  lateral  ‘‘ ganglionare  Grosszellen  ”  in  Lma. 
He  is  inclined  to  regard  them  as  a  widely  occurring  differen¬ 
tiation  of  the  ectoderm. 

In  a  subsequent  paper  (’9o)  Graber  describes  and  figures  a 
foliated  condition  of  the  ganglia  in  the  nerve-cord  of  Steno- 
bothnis.  In  Fig.  52  the  neuroblasts  are  distinctly  seen,  and  in 
one  lateral  cord  five,  in  the  other  four  pillars  of  cells  may  be 
distinguished.  So  far  as  the  neuroblasts  are  concerned,  he 
cannot  be  said  to  have  added  anything  to  Korotneff’s  account. 

Nusbaum  (’9i),  in  a  recent  Polish  paper  on  the  development 
of  Meloe\  figures  neuroblasts  in  the  lateral  cords.  They  are 
frequently  represented  in  mitosis  —  the  spindle-axes  being  in 
some  cases  perpendicular  to  the  surface  of  the  ganglia  (Figs. 
94,  95)  while  in  others  (Fig.  107)  they  are  parallel  to  the 
surface,  as  in  Do7yphora.  Such  portions  of  the  text  as  were 
translated  for  me  contained  nothing  new  on  these  structures. 

Viallanes,  in  two  recent  papers  (’90®  ’90^)  on  the  structure 
of  the  nervous  system  in  the  embryo  Mantis^  comes  to  con¬ 
clusions  agreeing  with  my  own,  which  were  arrived  at  inde¬ 
pendently.  His  observations  on  the  neuroblasts  may  be 
briefly  summarized  in  his  own  words  (’90^,  p.  293):  “A  Tori- 
gine  le  bourrelet  primitif  n’est  qu’un  simple  epaississement 
de  r exoderm e,  c’est-a-dire  une  region  de  ce  feuillet  dont 
les  cellules  sont  devenues  columnaires  et  ont  augmente  de 
volume.  Bientot  ces  cellules  se  multiplient  et  se  divisent  en 
deux  couches.  Tune  superficielle  {dermatoghie) ,  Tautre  profonde 
ga7iglioghie) .  A  une  periode  plus  ou  moins  tardive,  suivant  la 
region  consideree,  la  couche  des  cellules  dermatogenes  se 
separe  de  la  couche  des  cellules  gangliogenes  et  devient  I’hypo- 
derme.  Les  cellules  gangliogenes  en  se  multipliant  donnent 
naissance  aux  cellules  ga7zgli07i7iatres.'' 

Viallanes’  figures  do  not  show  a  regular  arrangement  of  the 
cells  budded  forth  from  the  neuroblasts,  and  he  has  not  de¬ 
scribed  the  neuroblasts  of  the  median  cord,  probably  because 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  97 


his  attention  was  concentrated  on  the  structure  of  the  brain. 
He  has  observed  the  degeneration  of  the  gangliogenic  cells,  or 
neuroblasts.  In  a  late  stage  (’90^,  p.  301),  he  says  Ils  montrent 
des  signes  evidents  de  decrepitude ;  beaucoup  des  cellules  gang- 
liogenes  ont  deja  disparu,  les  autres  sont  en  voie  d’atrophie.” 

Our  knowledge  of  the  median  cord  cannot  be  said  to  have 
made  much  advance  since  this  structure  was  first  described  by 
Hatschek  (’77).  While  all  writers  agree  that  it  originally  ex¬ 
tends  as  an  uninterrupted  structure  from  the  mouth  to  the 
anus,  there  is  wide  difference  of  opinion  respecting  the  ultimate 
fate  of  its  inter-  and  intraganglionic  portions.  Hatschek  (’77), 
Tichomiroff  (’82),  and  Korotneff  (’85)  maintain  that  the  inter- 
ganglionic  portions  remain  attached  to  the  integument  when  the 
nerve-cord  is  liberated  and  that  they  ultimately  disappear. 
Ayers  (’84)  on  the  other  hand  holds  that  the  whole  median 
cord  is  liberated  from  the  ectoderm,  but  does  not  affirm  that 
the  interganglionic  portions  form  a  constituent  part  of  the 
ganglia. 

Graber  (’90)  has  very  recently  come  to  a  conclusion  which 
differs  from  the  views  hitherto  advanced.  With  Ayers  he 
holds  that  the  interganglionic  portions  of  the  median  cord  are 
delaminated  from  the  ectoderm  along  with  the  intraganglionic 
portions,  but  he  goes  further  and  claims  (p.  103)  that 
das  Zellenmaterial  des  interganglionalen  Mittelstranges, 
zum  Theil  wenigstens  mit  den  Ganglien  vereinigt  wird,  oder 
mit  anderen  Worten,  dass  eine  Vergrosserung  des  ganglionalen 
Mitteltheiles  auf  Kosten  des  interganglionalen  erfolgt.” 

As  will  be  inferred  from  the  above  descriptive  paragraphs, 
I  hold  to  Hatschek’ s  view  that  the  interganglionic  portions 
of  the  median  cord  take  no  part  in  the  formation  of  the 
ganglia  but  are  drawn  out  from  between  the  connections  and 
constitute  a  portion  of  the  sternal  integument.  Graber’ s  re¬ 
searches  on  this  portion  of  the  nerve  cord  are  limited  to  the 
Coleoptera  and  as  the  insects  of  this  order  certainly  differ  to 
some  extent  from  the  Orthoptera  in  the  formation  of  the 
nervous  system,  I  have  no  grounds  for  doubting  the  correctness 
of  his  observations.  I  believe,  however,  that  Ayers’  account 
of  the  median  cord  in  CEcantJms  is  open  to  criticism.  After 


98 


WHEELER. 


[VOL.  VIII. 


clearly  implying  that  the  median  cord  is  set  free  from  the 
ectoderm  along  its  whole  extent  he  remarks  (p.  252):  “Be¬ 
tween  the  successive  pairs  of  ganglia  the  median  ingrowth 
atrophies,  and  at  the  time  of  the  closure  of  the  dorsal  wall  of 
the  body  there  is  seen  between  the  connecting  cords  of  two 
adjacent  pairs  of  ganglia,  a  small  triangular  or  cylindrical  mass 
of  cells,  concerning  the  fate  of  which  I  am  not  absolutely 
certain.  I  believe,  however,  that  they  go  to  form  a  part  of 
the  internal  skeleton.  The  chitinous  rods  in  the  thoracic 
region  to  which  the  muscles  of  the  legs  and  wings  are  attached 
probably  arise  from  the  remnants  of  the  median  invagination, 
but  in  the  abdominal  region  they  may  disappear  entirely  with¬ 
out  giving  rise  to  such  structures.”  If  I  understand  this 
passage  correctly,  Ayers  implies  that  the  chitinous  rods  are 
originally  interganglionic  portions  of  the  median  cord.  But  if 
this  is  the  case,  how  can  the  median  cord  separate  completely 
from  the  ectoderm  unless  we  are  to  suppose  that  there  is  a 
reunion  of  the  interganglionic  portions  with  the  integument 
to  form  the  endoskeletal  structures  }  The  chitinous  rods  are 
directly  continuous  with  the  chitin  of  the  integument  so  that 
until  observations  are  forthcoming  to  show  that  portions  of  the 
integument  can  loosen  and  pass  into  the  body-cavity  and  subse¬ 
quently  reunite  with  the  integument,  I  must  regard  Ayers’ 
account  as  inadequate. 

I  am  still  in  some  doubt  as  to  the  exact  origin  of  the  commis¬ 
sures.  Grassi  (’84),  Ayers  (’84),  Heider  (’89),  and  Graber  (’90), 
all  maintain  that  the  commissural  fibres  arise  from  the  median 
cord  cells.  A  priori,  there  are  no  reasons  why  the  daughter- 
cells  of  the  median  neuroblast  should  not  send  out  processes  to 
form  Punktsubstanz  and  thus  form  a  commissure.  From  the 
position  of  these  cells,  however,  I  regard  it  as  highly  improbable 
that  anything  but  the  posterior  commissure  could  be  formed  in 
this  way.  The  isolated  Punktsubstanz  masses  in  the  Coleopte- 
ran  median  cord  in  Graber’ s  and  H eider’s  figures  may  arise 
from  cells  equivalent  to  the  daughter-cells  of  the  median  neuro¬ 
blasts  of  the  Orthoptera.  It  is  very  improbable  that  the  der- 
matoblastic  cells  which  form  the  walls  of  the  median  cord  in 
the  region  of  the  anterior  commissure,  and  which  I  regard  as 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


99 


non-nervous,  should  take  part  in  forming  the  fibres  of  that 
structure. 

Regarding  the  origin  of  the  neurilemmata  in  insects,  there  is 
still  considerable  doubt.  The  inadequacy  and  inconsistency  of 
Nusbaum’s  observations  on  Blatta  (’83)  have  been  sufficiently 
pointed  out  by  Eisig  (’8?)  and  Korotneff  (’85).  Nusbaum  derived 
the  median  cord  (which,  by  the  way,  he  did  not  recognize  as  the 
median  cord)  from  the  entoderm,  and  compared  it  with  the 
vertebrate  chorda.  So  far  his  observations  and  conclusions 
were  erroneous,  but  he  derived  the  inner  and  outer  neurilemma 
from  the  cells  of  this  “chorda” — an  observation  which  agrees 
essentially  with  my  own. 

Korotneff’ s  view  that  the  neurilemmata  arise  from  migrant 
mesoderm  cells  has  not  been  confirmed  by  recent  writers, 
who  are  inclined  to  derive  these  envelopes  from  the  ectoderm 
(Heider,  ’89  ;  Graber,  ’9o).  Though  I  venture  to  say  that  my 
own  observations  are  somewhat  more  definite  than  those  hith¬ 
erto  published,  I  cannot  regard  them  as  in  any  way  final. 

2.  The  Bram. 

In  the  following  account  of  the  Xiphidium  brain,  I  shall  use 
the  nomenclature  employed  by  Viallanes  in  his  recent  papers 
(’90®,  ’90^),  since  his  studies  on  the  brain-development  of 
Mantis  religiosa  agree  very  closely  with  my  own.  Before 
passing  to  a  description  of  my  sections  I  would  refer  the 
reader  to*  the  diagrammatic  figure  (VII)  which  represents  the 
main  points  in  the  structure  of  the  embryonic  brain.  Here  it 
is  seen  that  the  ventral  nerve-cord  bifurcates  just  in  front  of 
the  mandibular  segment  and  passes  on  either  side  of  the 
mouth,  where  it  forms  two  successive  pairs  of  ganglia.  The 
posterior  of  these  {tci)  is  the  tritocerebriLm,  or  third  brain  seg¬ 
ment.  Its  two  halves  are  united  by  the  infraoesophageal  com¬ 
missure^  shown  in  the  figure  as  a  broad  white  band  connecting 
the  Punktsubstanz-masses  of  the  ganglia.  The  anterior  pair 
of  swellings  {dci)  constitute  the  deutocerebrum,  or  second  brain 
segment.  From  this  portion  the  antennae  are  innervated. 
Further  forward  the  deutocerebrum  passes  into  a  large  paired 


lOO 


WHEELER. 


[VOL.  VIII. 


supraoesophageal  mass,  the  protocerebnim^  or  first  brain  seg¬ 
ment,  which  constitutes  the  greater  portion  of  the  brain. 
Each  of  its  halves  may  be  separated  into  three  lobes  ;  the 
first,  or  outermost  lobe  i^pc^  forms  the  optic  ganglion  of 

the  larva  and  imago,  while  the  second  and  third  lobes  {pc~y 
pc  3)  ultimately  form  the  bulk  of  the  brain  proper.  The  third 
lobe  is  united  with  the  contralateral  lobe  by  the  broad  siipra- 
(ssophageal  commissure.  Such  is  the  structure  of  the 
Orthopteran  brain  reduced  to  its  simplest  terms.  It  may  now 
be  considered  a  little  more  in  detail. 

Like  the  nerve-cord,  of  which  it  is  simply  a  modified  portion, 
the  brain  arises  from  neuroblastic  cells.  These  first  make 
their  appearance  in  clusters  (the  spots  seen  on  the  procephalic 
lobes  in  Fig.  2).  Later  they  form  a  single  layer  of  proliferat¬ 
ing  centres  continuous  with  and  in  every  way  comparable  to  the 
neuroblasts  of  the  ventral  nerve-cord.  Like  the  latter  they  are 
covered  externally  by  a  layer  of  dermatoblastic  cells. 

That  the  deuto-  and  tritocerebral  ganglia  are  strictly 
homodynamous  with  the  ganglia  of  the.  nerve-cord  is  clearly 
shown  in  Xiphidium.  In  the  first  place  these  brain  segments 
are  directly  continuous  with  the  segments  of  the  cord;  second, 
they  have  at  first  the  same  size  and  shape  as  the  latter,  and 
third,  they  present  on  the  average  four  neuroblasts  in  cross- 
section  on  either  side.  The  suppression  of  the  median  cord  in 
the  deutocerebrum  (if  it  be  not  drawn  forward  into  the  proto¬ 
cerebrum),  is  perhaps  sufficiently  explained  by  the  presence  of 
the  stomodaeal  invagination.  A  partial  suppression  of  the 
median  cord  in  the  tritocerebrum  may  be  due  to  the  same 
cause.  The  infraoesophageal  commissure  is  perhaps  the 
morphological  equivalent  of  both  the  commissures  of  a  ventral 
ganglion. 

The  early  clustered  condition  of  the  neuroblasts  is  seen  in 
Figs.  32-34  at  7tb.  At  the  edges  of  these  cross-sections  a 
rounded  mass  of  pale  cells  (^pc^  distinctly  marked  off 

from  a  more  deeply  stainable  layer  which  encloses  it  on  nearly 
all  sides.  This  mass,  the  future  optic  ganglion  (first  proto¬ 
cerebral  lobe),  is  delaminated  from  the  ectoderm  at  a  very 
early  stage.  The  cells  of  the  mass  agree  with  the  neuroblasts 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


lOI 


in  their  slight  affinity  for  stains;  they  differ  in  the  more 
elongate  shape  of  their  nuclei  and  cytoplasm.  The  dark  layer 
enclosing  the  optic  ganglion  on  all  except  its  innermost  face  is 
the  optic  plate  and  will  give  rise  to  the  compound  eye.  Passing 
towards  the  median  line  in  these  sections  (especially  in  Fig.  33) 
two  other  thickenings  may  be  distinguished  pc^)  —  the 
second  and  third  protocerebral  lobes. 

The  cross-section,  Fig.  36,  runs  through  the  labrum  of  an 
older  embryo  (Stage  F)  and  shows  a  considerable  advance  in 
the  structure  of  the  brain.  The  three  protocerebral  lobes  are 
distinctly  marked  out.  In  the  second  and  third,  the  neuro¬ 
blasts  have  arranged  themselves  in  a  row  and  have  budded 
forth  strings  of  ganglion  cells.  In  the  first  lobe  (/<;i  [o.^.])  no 
teloblastic  arrangement  is  ever  present.  The  cells  are  small 
and  narrow  and  early  assume  a  radial  arrangement  around  the 
Punktsubstanz  core  at  the  base  of  the  mass.  The  cells  of  the 
optic  plate,  which  stand  away  from  the  surface  of  the  ganglion, 
already  show  a  tendency  to  differentiate  in  that  they  have 
become  smaller  and  narrower  than,  the  dermatoblasts  covering 
the  two  other  lobes  of  the  protocerebrum.  At  the  juncture  of 
the  second  with  the  third  lobe,  several  large  dermatoblastic 
cells  are  intercalated  (Ir'I.).  They  are  continuous  with  the 
dermatoblasts  covering  the  second  protocerebral  lobe.  This 
intercalated  mass  is  called  by  Viallanes  the  bourrelet  ectoderm- 
ique  intraganglionnaire .  I  shall  call  it  the  intraganglionic 
thickening. 

A  still  more  advanced  stage  in  the  development  of  the  brain 
is  seen  in  Fig.  37  (Stage  G).  This  section  passes  above  the 
base  of  the  labrum.  Owing  to  the  active  proliferation  of  the 
neuroblasts,  the  mass  of  the  protocerebrum  is  greatly  aug¬ 
mented.  The  Punktsubstanz  has  made  its  appearance  as  a 
confluent  mass.^  The  optic  plate  is  much  thickened  and  its 
small  cells  are  about  to  arrange  themselves  to  form  the 
ommatidia.  The  intraganglionic  thickening  {igll)  presents 
an  interesting  appearance.  The  edge  of  the  optic  plate  is 
united  with  the  inner  edge  of  the  optic  ganglia,  but  between  it 

1 1  have  seen  nothing  to  corroborate  Cholodkowsky’s  view  (’91)  that  there  are 
at  first  three  distinct  and  separate  pairs  of  Punktsubstanz  masses  in  the  brain. 


102 


WHEELER. 


[VoL.  VI I L 


and  the  second  protocerebral  lobe  there  is  a  fissure  {w)  which 
extends  in  some  distance.  Examination  of  a  number  of  succes¬ 
sive  sections  and  stages  has  convinced  me  that  this  fissure  is 
not  the  result  of  artificial  rupture  during  sectioning  but  that  it 
is  brought  about  by  an  infolding  of  the  intraganglionic  thicken¬ 
ing.  The  shape  and  position  of  the  involuted  mass  may  be 
clearly  seen  from  the  surface  in  Stages  H  and  I  (see  Figs.  8 
and  9,  igl). 

In  the  frontal  section  (Fig.  39)  are  shown  the  relations  of 
the  protocerebrum  to  the  outer  brain-segments  and  to  the 
ventral  cord.  Only  a  small  portion  of  the  optic  plate  (e)  is 
cut.  Beneath  it  lies  the  optic  ganglion  (/<;i  [<^g-]))  the  small 
cells  of  which  contrast  with  the  large  cells  of  the  brain  proper. 
The  second  protocerebral  lobe  (pc^)  still  contains  many  neuro¬ 
blasts  which  are  budding  forth  their  last  progeny.  The  older 
daughter-cells  have  already  assumed  an  irregular  arrangement. 
The  brain  is  separated  from  the  attenuated  dermatoblastic,  or 
integumental  layer  (dd.)  beneath  which  the  outer  neurilemma 
(enl.)  is  forming.  The  inner  neurilemma  (ml.)  envelops  the 
Punktsubstanz  portions  of  the  brain.  The  broad  supraoesoph- 
ageal  commissure  (s.  cm)  connects  the  third  protocerebral  lobes 
of  the  two  sides.  As  shown  in  the  figure,  the  deutocerebrum 
is  distinctly  praeoral.  At  an  is  shown  the  point  where  the 
antennary  nerve  leaves  the  fibrous  portion  of  this  brain  seg¬ 
ment.  Caudad  to  the  deutocerebrum  lies  the  tritocerebrum, 
a  pair  of  somewhat  smaller  ganglia  united  by  the  infraoesoph- 
ageal  commissure.  It  is  this  segment  which  according  to 
Viallanes  innervates  the  labrum  and  the  frontal  ganglion.  Be¬ 
sides  the  supra-  and  infraoesophageal  commissures  and  the 
connectives  which  arise  in  the  third  protocerebral  lobes  and 
traverse  the  deuto-  and  tritocerebrum  to  pass  into  the  man¬ 
dibular  ganglion  and  thence  through  the  nerve-cord,  I  may  call 
attention  to  two  other  masses  of  Punktsubstanz  which  lie  in 
front  of  the  supraoesophageal  commissure.  These  are  shown 
at/,  in  the  figure.  They  appear  to  be  connected  by  a  small 
band  running  parallel  to  the  robust  supraoesophageal  commis¬ 
sure.  I  did  not  succeed  in  finding  these  connected  Punktsub¬ 
stanz  masses  in  all  embryos  of  this  stage,  and  as  they  were  not 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  103 


seen  in  later  stages,  their  morphological  value  as  indicating  the 
presence  of  another  segment  in  front  of  the  protocerebrum  is 
somewhat  doubtful. 

The  three  divisions  of  the  protocerebrum  may  still  be  rec¬ 
ognized  in  the  transverse  section  (Fig.  40)  of  an  embryo  which 
has  passed  beyond  Stage  J.  In  the  median  line  at  me  lies  a 
rounded  and  compact  mass  of  cells  which  I  regard  as  the  prae- 
oral  representative  of  the  median-cord  cell-masses  of  the  ventral 
cord.  A  large  cell,  which  I  take  to  be  a  neuroblast,  lies  at  the 
outer  periphery  of  this  median  cell-mass  at  a  younger  stage. 
Structures  at  a  corresponding  position  in  the  brains  of  other 
insects  {Hydrophilus^  Mused)  and  likewise  comparable  to  the 
median-cord  have  been  described  by  Heider  (’89)  and  Graber 
(’89,  p.  49).  It  is  not  improbable  that  the  brain  neurilemmata 
may  have  their  origin  near  this  median  cerebral  mass,  just  as 
the  neurilemmata  of  the  ventral  cord  probably  arise  from  the 
non-ganglionic  portions  of  the  neural  furrow. 

Two  other  interesting  structures  are  shown  in  Fig.  40:  the 
intraganglionic  thickening  {igl)^  now  completely  separated  from 
the  integument  and  lying  as  a  deeply  stainable  mass  wedged  in 
between  the  optic  ganglion  and  the  second  protocerebral  lobe, 
and  a  peculiar  pale  thickening  at  the  edge  of  the  optic  plate  {tJi). 

In  later  stages  it  is  very  difficult  to  locate  the  intraganglionic 
mass,  so  that  I  am  unable  to  decide  whether  it  atrophies  or 
persists  in  a  modified  form  as  a  portion  of  the  brain.  The  re¬ 
searches  of  Viallanes  on  Mantis  would  seem  to  lend  great 
probability  to  the  former  alternative. 

The  thickening  at  the  lateral  edge  of  the  optic  plate  is  con¬ 
stant  in  Stages  G  to  J  and  somewhat  later.  It  soon  entirely 
disappears,  without  taking  any  part  in  the  formation  of  the  eye, 
so  far  as  I  have  been  able  to  observe.  Does  it  represent  an 
abortive  ocellus  ? 

The  fully  established  optic  nerve  is  shown  in  Fig.  41  ((?.  n). 
In  a  much  earlier  stage  it  may  be  found  as  a  delicate  band  of 
cells  connecting  the  posterior  edge  of  the  optic  ganglion  with 
the  optic  plate  (Fig.  38,  0.  n).  I  agree  with  Viallanes,  that  it 
seems  to  arise  from  the  ganglion  and  to  grow  outwards  into  the 
ommatidial  layer;  for  there  is  from  the  first  a  sharply  defined 


104 


WHEELER. 


[VOL.  VIII. 


intercepting  membrane  between  the  nerve  and  the  plate,  whereas 
the  nerve  passes  without  interruption  into  the  ganglion.  But  I 
am  led  to  lay  little  stress  on  these  appearances  by  the  researches 
of  Watase  (’90)  and  Parker  (’90)  on  the  adult  ommatidia  of  a 
number  of  Arthropods.  They  have  shown  in  a  very  convinc- 
ino"  manner  that  each  retinula-cell  is  the  termination  of  an 
optic  nerve  fibre.  The  retinulae  are  undoubtedly  modified  optic 
plate  cells;  and  judging  from  recent  observations  on  per¬ 
cipient  cells  in  other  forms  (vertebrate  eye,  ear,  olfactory 
nerves,  v.  Lenhossek’s  observations  on  Liimb7'icus  (’92),  etc.), 
we  must  suppose  that  the  nerve  fibres  grow  out  from  the 
retinula-cells,'  pass  through  the  optic  nerve  and  enter  the 
ganglion.  Such  prolongations  from  all  the  retinulae  would  be 
amply  sufficient  to  form  the  optic  nerve,  although  it  is  probable 
that  some  of  its  fibres  are  centrifugal  prolongations  from  optic 
ganglion  cells.  It  has  been  suggested  that  the  optic  nerve 
may  be  established  at  a  very  early  stage,  when  the  optic  plate 
and  optic  ganglion  are  still  closely  applied  to  each  other  {vide 
Figs.  32-34),  and  that  the  nerve  may  not  become  visible  till 
the  two  Anlagen  separate  with  further  development.  But  I  do 
not  think  this  is  the  case  in  Xiphidhim.  Sections  like  Fig. 
36  show  a  distinct  separation  of  the  optic  plate  and  ganglion 
in  the  region  of  the  future  optic  nerve;  and  Viallanes  has  made 
an  exactly  similar  observation  on  Mantis. 

The  sympathetic  nervous  system  arises  in  part  at  least  from 
the  dorsal  median  wall  of  the  oesophagus.  At  three  separate 
points  (Fig.  61)  the  ectoderm  becomes  thickened  and  its  outer 
cells  enlarge  and  assume  the  character  of  ganglion  cells.  The 
most  anterior  of  these  thickenings  (^f.  g.)  is  the  frontal  gang¬ 
lion.  It  arises  just  behind  the  base  of  the  labrum.  The  two 
other  thickenings  which  are  placed  further  back  {rg^y  rg^)  are 
the  second  and  third  visceral  ganglia.  I  have  not  followed  the 
development  of  the  nerves  which  unite  these  ganglia  and 
ramify  from  them. 

Concerning  the  origin  of  the  peripheral  nervous  system  I 
have  no  positive  data.  In  a  few  cases  I  have  seen  appearances 
which  led  .me  to  believe  that  they  arise  as  outgrowths  from 
their  respective  ganglia. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  105 

The  development  of  the  brain  of  Blatta  germajiica  and 
Melanoplus  feimtr-riibrimi  agrees  in  all  essential  respects  with 
the  development  of  the  Xiphidium  brain.  Certain  Hemiptera, 
e.g.  Ranatra  fusca^  conform  very  closely  to  the  type  of  brain 
structure  seen  in  these  Orthoptera.  I  may  mention  in  this 
connection  that  the  brain  of  Anurida  niaritima  shows  the 
typical  division  into  proto-,  deuto-  and  tritocerebral  segments 
with  great  distinctness.  The  last  segment  especially  is  re¬ 
markably  distinct. 

Until  very  recently  the  detailed  study  of  the  embryonic 
Hexapod  brain  has  been  limited  to  the  Coleoptera  and  the 
results  obtained  have  been  naturally  enough  extended  to 
include  not  only  other  insects  but  other  Arthropods  as  well. 
The  Coleoptera,  however,  are  far  from  being  primitive  forms 
and  the  role  which  they  play  in  contemporary  embryological 
literature  is  largely  attributable  to  the  unusual  technical  ad¬ 
vantages  presented  by  their  eggs.  As  far  as  development  is 
concerned,  the  simpler  brain  of  the  Orthoptera  and  Ametabola 
in  general  offers  many  points  of  resemblance  to  the  Crustacea 
and  Myriopoda,^  whereas  the  brain  of  the  Metabola,  like  so 
many  other  points  in  their  organization  bears  witness  to  a 
considerable  amount  of  modification.  It  is  therefore  more 
consistent  with  our  general  views  of  phylogeny  to  reduce  the 
Coleopteran  brain  to  the  Orthopteran  type  than  to  proceed 
vice  versa. 

We  owe  the  most  important  contributions  to  the  subject  of 
Orthopteran  brain  development  to  Viallanes.  After  a  decade 
of  study  devoted  to  the  histological  structure  of  the  adult 
Arthropod  brain  he  has  selected  Mantis  as  a  subject  for 
embryological  investigation.  His  previous  careful  study  of 
the  adult  brain  of  other  Orthoptera  {CEdipoda  coernlescens 
and  Caloptenus  italicus,  ’87^)  has  enabled  him  to  avoid  the  con¬ 
fusion  with  which  the  inexperienced  investigator  is  overwhelmed 
when  attempting  to  follow  the  rapidly  increasing  complication 
of  neural  structures.  With  his  usual  skill  and  patience  he  has 
traced  the  development  not  only  of  the  main  structural  features 
but  of  many  details,  so  that  we  have  a  well-established  point 

1  See  the  papers  of  St.  Remy  (’90)  and  Viallanes  (’87®,  ’87^). 


io6 


WHEELER. 


[VOL.  VIII. 


of  departure  for  further  comparative  studies.  So  far  as  my 
own  observations  are  concerned  I  am  able  to  corroborate 
Viallanes’  results  on  nearly  all  important  points.  I  must  state, 
however,  that  I  have  not  followed  the  development  into  such 
detail. 

In  the  light  of  these  researches  a  reconsideration  of  the 
Coleopteran  brain  must  be  undertaken.  Patten’s  description 
of  the  Acilius  brain  (’88)  and  my  description  of  the  brain  of 
Doryphora  (’89)  need  revision  and  alteration.  We  described 
the  organ  (see  my  Fig.  72,  PL  XIX)  as  consisting  of  three 
segments,  each  of  which  was  subdivided  into  a  brain  portion, 
continuous  with  the  ganglia  of  the  ventral  cord,  an  optic 
ganglion  portion  and  an  optic  plate  portion.  Between  the 
third  brain  and  the  mandibular  segment,  a  segment  was  found 
which  I  designated  as  intercalary.  This  segment  is  also 
clearly  seen  in  some  of  Patten’s  figures  (Figs.  2  and  2"  s4  PI. 
VII).  Thus  according  to  our  account  there  were  four  preman- 
dibular  segments  or  seven  segments  in  the  entire  head.  Our 
results  were  obtained  almost  exclusively  from  surface  views  — 
by  itself  a  defective  method.  But  greater  error  was  incurred, 
it  seems  to  me,  in  ascribing  segmental  values  to  the  various 
prominences  of  the  optic  ganglion  and  optic  plate. 

In  order  to  bring  our  observations  into  harmony  with  the 
results  obtained  from  a  study  of  the  Orthopteran  brain,  our 
figures  must  be  interpreted  in  a  very  different  way  from  that 
in  which  we  chose  to  interpret  them.  Our  first  brain-segment 
is  probably  no  segment  at  all,  but  merely  a  slight  elevation 
often  seen  near  the  median  line  at  the  extreme  anterior  end  of 
the  germ-band.  Our  second,  third  and  intercalary  segments 
are  equivalent  to  Viallanes’  third  protocerebral  lobe,  deuto- 
cerebrum  and  tritocerebrum.  The  three  divisions  of  the  optic 
ganglion  are  not  parts  of  three  segments,  but  the  whole  struc¬ 
ture  belongs  to  the  protocerebrum,  of  which  it  forms  the 
first  lobe.  In  the  same  way  we  cannot  regard  the  optic 
plate  as  trisegmental  since  it  has  no  connection  with  the 
deuto-  and  tritocerebrum  but  only  with  the  optic  ganglion.  It 
follows  that  the  ocelli  of  Coleoptera  are  not  originally  formed 
on  different  segments  as  Patten  would  have  us  believe,  but 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  107 


belong  to  one  segment  —  the  protocerebrum.  That  such  is 
the  case  I  am  convinced  from  a  study  of  the  eyes  in  embryos 
of  Dytiscus  verticalis,  a  form  closely  related  to  Acilius. 

As  will  be  seen  in  the  profile  view  Fig.  8,  the  optic  ganglion 
and  optic  plate  of  Xiphiditini  are  at  first  folded  back  so  as  to 
lie  along  side  the  deuto-  and  tritocerebrum.  The  antennal 
furrow  runs  forward,  separating  the  optic  ganglion  from  the 
brain  but  stops  when  it  reaches  the  protocerebrum.  The 
value  of  this  furrow  as  completely  separating  the  second  and 
third  brain-segments  from  the  optic  ganglion  was  overlooked 
by  Patten  and  myself:  hence  our  false  interpretation  of  the 
structures  lying  laterad  to  it. 

I  believe  that  I  am  justified  in  putting  this  new  interpreta¬ 
tion  on  the  Coleopteran  brain,  because  it  harmonizes  with 
H eider’s  careful  study  of  Hydrophihcs  (’89).  He  has  failed  to  find 
indications  of  segmental  constrictions  in  the  optic  plate  and 
optic  ganglion  and  his  figure  4  A.  B.  at  p.  37  agrees  closely 
with  Viallanes’  description  of  the  Mantis  brain.  It  should  be 
observed  that  the  embryos  of  Hydrophihis  are  much  larger 
than  those  of  Aciliits  and  Doiyphora  and  therefore  much  more 
favorable  for  surface  study.  On  the  other  hand  it  may  be 
urged  that  Heider  evidently  did  not  employ  so  good  a  method 
of  surface  preparation  as  Patten. 

The  distinct  invagination  associated  with  the  formation  of 
the  optic  ganglion  in  Coleoptera  and  described  by  Patten, 
Heider  and  myself,  is  probably  homologous,  as  Viallanes  sug¬ 
gests,  with  the  intraganglionic  thickening  of  the  Orthop- 
teran  embryo.  This  structure  in  Ma7ttis,  and  probably  also 
in  Xiphidmin^  takes  no  part  in  the  formation  of  the  optic 
ganglion,  which  arises  —  at  least  in  great  part  —  by  delamina¬ 
tion  as  in  the  Crustacea  (see  Parker  (’90)).  Only  the  outer  or 
lateral  portion  of  the  optic  plate  becomes  the  compound  eye, 
so  that  in  a  later  stage  the  intraganglionic  thickening  is  separ¬ 
ated  from  the  edge  ©f  the  eye  by  a  considerable  space.  The 
thickening  then  lies  just  laterad  to  the  antennal  furrow  as 
shown  in  Fig.  8.  Whether  or  not  the  invagination  in  the 
Coleoptera  really  plays  any  part  in  forming  ganglionic  tissue 
as  has  been  claimed,  must  be  decided  by  renewed  investi¬ 
gations. 


io8 


WHEELER. 


[VOL.  VIII. 


Concerning  the  researches  of  Cholodkowsky  on  the  brain  of 
Blatta  germa7iica,  I  must  say  a  few  words,  since  I  have  de¬ 
scribed  the  brain  and  nerve-cord  of  this  form  as  agreeing  in  all 
essential  respects  with  those  of  Xiphiditmi.  Cholodkowsky  lays 
great  stress  on  the  existence  of  three  distinct  pairs  of  Punkt- 
substanz  masses  in  the  supraoesophageal  ganglion  as  indicat¬ 
ing  the  presence  of  three  segments.  When  we  come  to  exam¬ 
ine  his  figures  we  find  that  he  takes  a  very  unusual  view  of 
brain-segmentation,  for  the  three  pairs  of  Punktsubstanz  masses 
are  seen  to  belong  (Fig.  46  PI.  IV;  Fig.  67  PI.  VI)  to  the 
protocerebrum  and  correspond  to  the  centres  of  its  three  lobes. 
He  did  not  distinguish  the  deuto-  and  tritocerebral  segments  ! 
Such  of  his  remarks  on  the  development  of  the  brain  and 
ventral  nerve-cord  as  are  at  all  comprehensible  show  similar 
glaring  defects  in  observation.  Thus  he  has  failed  to  detect 
the  small  dermatoblastic  cells  which  from  the  first  cover  the 
brain  and  nerve-cord.  He  asserts  that  these  organs  are  at  first 
naked  and  are  only  subsequently  covered  by  an  overgrowth  of 
the  integument  from  the  sides  of  the  body.  The  antennal  and 
neural  furrows  do  not  play  the  part  in  development  that  he 
ascribes  to  them.  The  last  abdominal  ganglion  of  the  mature 
embryo  does  not  consist  of  four  but  of  three  fused  ganglia; 
the  fusion  of  the  second  and  third  abdom’inal  ganglia  was  com¬ 
pletely  overlooked. 

3.  G enteral  Remarks  on  the  Nervoiis  System. 

The  nervous  system  of  Arthropods  is  by  common  consent 
derived  from  the  nervous  system  of  annelid-like  forms,  and  it 
is  to  this  group  that  we  naturally  turn  in  seeking  an  explanation 
for  certain  structures  in  the  Hexapod  brain  and  nerve-cord. 

In  a  brief  preliminary  paper  Patten  (’88)  made  the  statement, 
that  “  the  ventral  cord  and  brain  of  Arthropods  is  at  first 
composed  entirely  of  minute  sense-organs,  which  in  scorpions 
have  the  same  structure  as  the  segmental  ones  at  the  base  of 
the  legs.”  This  would  seem  to  indicate  that  the  Arthropod 
nervous  system  can  be  traced  back  to  the  condition  seen  in 
Polychseta  —  Lopadorhynchus ,  according  to  Kleinenberg  (’86)  — 
where  both  brain  and  nerve-cord  arise  in  connection  with  and 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


ultimately  supplant  certain  larval  sense-organs.  So  far,  how¬ 
ever,  as  the  Hexapoda  are  concerned,  Patten’s  statement  is,  to 
say  the  least,  inapposite,  since,  as  I  have  pointed  out,  both 
brain  and  nerve-cord  arise  from  peculiar  ectodermal  cells  — 
the  neuroblasts  —  which  under  no  circumstances  can  be  re¬ 
garded  as  primitively  sensory.  They  are  simply  generalized 
cells,  like  the  teloblasts  of  worms  and  the  meristem  of  plants. 

The  development  of  the  nerve-cord  in  the  Hirudinea  and 
Oligochaeta  agrees  more  closely  with  the  conditions  seen  in  in¬ 
sects.  As  Whitman  has  shown  for  Clepsine  (’87),  and  E.  B.  Wil¬ 
son  for  Lumbricus  (’83),  the  nerve-cord  is  proliferated  forward 
from  a  pair  of  neuroteloblasts  situated  at  the  posterior  end  of 
the  germ-band.  Hence,  in  these  worms,  the  whole  of  the  nerve- 
cord  is  condensed,  as  it  were,  into  two  huge  mother-cells, 
whereas  in  the  Insecta  it  is  condensed  into  a  single  layer  of 
huge  cells.  There  are  reasons,  however,  for  believing  that 
this  layer  is,  in  part  at  least,  derived  from  a  few  large 
cells  situated  just  in  front  of  the  anus,  and  therefore  corre¬ 
sponding  to  the  Annelid  neuroteloblasts.^  That  there  are  only 
two  rows  of  these  cells  in  Annelids,  while  there  are  eight  in 
insects,  can  form  no  very  serious  objection  to  their  homology, 
as  I  pointed  out  in  my  preliminary  note  (’90°). 

Certain  conditions  in  the  Crustacea  also  lend  probability  to 
the  view  that  the  Hexapod  neuroblasts  may  be  budded  forth 
from  a  prae-anal  row  of  teloblasts.^  Patten  (’9o)  has  pointed  out 
in  Cyniothoa  a  row  of  proliferating  cells  which  form  ectoderm, 
and  Nusbaum  (’9i)  has  described  a  very  similar  condition  in 
Ligia.  I  have  observed  the  same  phenomenon  in  Porcellio, 
and  believe  it  to  be  of  general  occurrence  throughout  the 
Isopoda.  The  cells  are  budded  forth  so  as  to  form  regular 
transverse  and  longitudinal  rows.  Reichenbach  describes  and 


1  What  I  have  called  the  neuroblasts  in  insects  therefore  correspond  to  the 
“  neural  cell-rows  ”  in  Annelids  (the  cells  np.  c.  in  E.  B.  Wilson’s  Fig.  59,  PI.  XIX, 
and  the  cells  nc.  in  Whitman’s  Figs.  9  and  ii,  PI.  V). 

2  The  neuroblasts  of  the  last  row  {tb.,  Fig.  56,  PI.  VI)  in  the  nerve-cord  of 
Xipkidium  are  always  distinctly  larger  and  clearer  than  the  neuroblasts  of  the 
remainder  of  the  cord.  They  may  be  true  neuroteloblasts  and  give  rise  to  the 
neuroblasts,  but  as  I  have  never  found  unmistakable  caryokinetic  figures  in 
them,  I  am  still  in  doubt  as  to  their  homology  with  the  neuroteloblasts  of  worms. 


I  lO 


WHEELER. 


[VOL.  VIII. 


figures  a  very  similar  budding-zone  of  ectoderm  cells  in  the 
Decapod  Astacus  (’86).  That  a  portion  of  the  ectoderm  cells 
thus  budded  forth  in  these  forms  goes  to  form  the  nervous 
system,  admits,  it  seems  to  me,  of  very  little  doubt.  The 
clustered  condition  of  the  neuroblasts  in  the  young  germ-band 
of  Xiphidium  may  be  due  to  the  rapidity  with  which  the 
germ-band  grows  in  length  and  breadth;  the  original  regular 
arrangement  of  the  cells  being  thereby  disturbed  and  not 
re-established  till  a  somewhat  later  stage. 

Although  I  have  maintained  a  phylogenetic  connection  of 
the  insect  neuroblasts  with  the  neural  cell-rows  of  Annelids,  I 
admit  that  they  may  be  regarded  from  an  entirely  different 
standpoint,  viz.  as  having  arisen  independently  in  insects  by 
a  process  of  precocious  segregation  ;  but  it  should  be  noted 
in  this  connection  that  it  is  just  the  oldest  Pterygota,  the 
Orthoptera,  which  show  this  segregation  most  clearly,  while  in 
the  more  recent  forms  (Coleoptera,  etci)  the  process  is  more 
obscure. 

A  comparison  of  the  histogenesis  of  the  insectean  with  the 
histogenesis  of  the  vertebrate  nervous  system  brings  out  some 
interesting  analogies.  The  neuroblasts  may  be  compared  with 
His’  Keimzellen  which  divide  by  caryokinesis  to  form  his  neuro¬ 
blasts.  These  are  directly  converted  into  ganglion-cells  by 
sending  out  axis-cylinder  processes.  They  correspond,  there¬ 
fore,  to  the  daughter-cells  of  the  insect  neuroblasts,  which 
are  likewise  converted  into  ganglion-cells.  The  “Keimzellen  ” 
of  vertebrates  differentiate  close  to  the  central  canal,  which  is, 
of  course,  morphologically  the  outer  surface  of  the  cord,  just  as 
the  Arthropod  neuroblasts  lie  at  the  surface  of  the  cord.  In 
both  groups  the  daughter-cells  appear  to  be  budded  off  in  the 
same  direction  morphologically;  though  in  vertebrates  the  gan¬ 
glion-cells  migrate,  while  in  insects  they  are  pushed  inwards  by 
their  newly  proliferated  sister-cells.  The  early  separation  of 
the  neural  ectoderm  in  vertebrates  into  Keimzellen  and  susten- 
tacular  tissue  (spongioblasts  of  His)  is  paralleled  in  insects  by 
the  precocious  splitting  of  the  same  germ-layer  into  neuro¬ 
blasts  and  dermatoblasts,  the  latter  giving  rise  to  supporting 
tissue  in  the  form  of  neurilemmata. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  m 


The  majority  of  authors  hold  that  the  Arthropod  brain  is 
either  wholly  or  in  part  homologous  with  the  Annelid  brain. 
Patten  (’90)  alone  takes  the  view  that  the  Annelid  prostomium 
is  absent  in  Arthropods,  and  that  the  brain  of  the  latter  is 
formed  by  the  moving  forward  of  three  segments  which  are 
postoral  in  the  Annelids.  Apart  from  the  fact  that  we  have  as 
yet  no  means  of  deciding  whether  what  we  call  the  first  seg¬ 
ment  of  the  Arthropod  head  (protocerebrum)  is  really  a  single 
segment  or  a  complex  of  several,  it  is  extremely  improbable 
that  so  highly  important  a  structure  as  the  Annelid  brain  should 
have  completely  disappeared  in  the  Arthropods.  So  great  is 
the  resemblance  between  the  Arthropods  and  Annelids  in  all 
the  more  important  morphological  features  and  even  in  the  de¬ 
tailed  structure  of  the  ventral  nerve-cord,  that  the  complete 
elimination  of  the  brain  certainly  makes  strong  demands  on 
one’s  credulity. 

Will  (’88)  goes  to  the  opposite  extreme  and  regards  the 
praeoral  portion  of  the  Arthropod  brain  as  the  exact  homologue 
of  the  Annelid  brain.  He  goes  so  far  as  to  call  the  procephalic 
lobes  of  Aphis  the  “  Scheitelplatte.”  He  finds  that  they  lie  at 
the  pole  of  the  egg  opposite  the  blastopore,  or  rather  what 
he  takes  to  be  the  blastopore,  and  that  they  arise  independently 
of  the  nerve-cord.  Now  the  “  Scheitelplatte  ”  of  Aphis  must 
include  at  least  the  proto-  and  deutocerebral  segments  —  prob¬ 
ably  also  the  tritocerebrum.  The  deutocerebrum  in  all  the 
Orthoptera  which  I  have  examined  is  provided  with  a  pair  of 
true  mesodermic  somites  and  with  a  pair  of  appendages,  the 
antennae.  Each  mesodermal  somite  sends  a  hollow  diverticu¬ 
lum  into  an  antenna,  which  is  thus  shown  to  be  homodynamous 
with  the  other  appendages  of  the  embryo.  The  tritocerebral 
segment  also  contains  a  pair  of  abortive  somites  and  in  Anurida 
maritima,  as  I  have  lately  ascertained,  bears  a  pair  of  minute 
but  distinct  appendages  (see  Fig.  VI,  tc.  ap.).  Viallanes  (’87®) 
and  St.  Remy  (’9o)  have  found  that  the  second  pair  of  antennae 
in  Crustacea  belong  to  the  tritocerebral  segment.  These  facts 
go  to  show  that  the  deuto-  and  tritocerebral  segments  are  homo¬ 
dynamous  with  the  postoral  segments  and,  as  the  “  Scheitel- 
platte  ”  of  Aphis  must  extend  at  least  as  far  back  as  the 


I  12 


WHEELER. 


[VOL.  VIII. 


tritocerebral  segment,  it  cannot  be  homologized  with  the 
Annelid  Scheitelplatte,  a  structure  which  is  not  segmented. 

A  view  midway  between  Will’s  and  Patten’s  probably  accords 
best  with  the  facts  at  our  disposal.  The  Arthropod  proto¬ 
cerebrum  probably  represents  the  Annelid  supraoesophageal 
ganglion,  while  the  deuto-  and  tritocerebral  segments, 
originally  postoral,  have  moved  forward  to  join  the  primitive 
brain.  This  is  essentially  Lankester’s  view  (’8i),  according  to 
which  in  Arthropods  ^-'the  prseoesophageal  ganglion  is  a  syn- 
cerebrum  consisting  of  the  archicerebrum  and  of  the  ganglion 
masses  appropriate  to  the  first  and  second  pair  of  appendages 
which  were  originally  postoral,  but  have  assumed  a  praeoral 
position  whilst  carrying  their  ganglion-masses  up  to  the  archi¬ 
cerebrum  to  fuse  with  it.” 

In  comparing  the  Arthropod  with  the  Annelid  brain  much 
stress  has  been  laid  on  the  fact  so  clearly  brought  out  by 
Kleinenberg  (’86)  —  that  the  Annelid  supraoesophageal  ganglion 
originates  independently  of  the  ventral  nerve-cord.  Several 
investigators  —  Balfour  (’80),  Schimkewitch  (’87),  Will  (’88)  and 
others  —  have  fancied  that  they  could  detect  a  similar  onto¬ 
genetic  discontinuity  of  the  brain  and  nerve-cord  in  Arthropods. 
But  more  recent  observations  all  tend  to  prove  that  there  is 
a  direct  continuity  of  the  central  nervous  system  from  the  time 
when  the  ganglia  first  make  their  appearS.nce.  So  far  as  the 
insects  are  concerned  I  may  note  that  Will’s  conclusions  were 
based  on  the  defective  surface  observation  of  a  form  {Aphis)  ill 
adapted  to  the  study  of  the  central  nervous  system.^ 

Even  granting  that  the  Annelid  brain  arises  independently  of 
the  nerve-cord  —  and  this  is  not  yet  settled  —  at  least  so  far  as 
the  Oligochaeta  are  concerned  (see  E.  B.  Wilson,  ’89)  —  Lankes¬ 
ter’s  view  of  the  Arthropod  brain  is  in  no  way  invalidated.  The 
line  of  separation  corresponding  to  the  Annelid  prototroch  must 
fall  in  front  of  the  deutocerebral  segment,  since  it  has  been 
shown  that  this  segment  in  some  insects  contains  a  pair  of  well- 

1  Little  value  can  be  attached  to  Cholodkowsky’s  assertion  that  in  Blatta  the 
supraoesophageal  ganglion  originates  independently  of  the  nerve-cord,  since  he  has 
failed  to  see  the  deuto-  and  tritocerebral  segments  which  are  quite  as  well 
developed  in  Blatta  as  in  other  Orthoptera. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  113 

developed  mesodermic  somites.  When  we  stop  to  consider  the 
intimate  union  of  the  proto-  and  deutocerebral  ganglia  from 
the  time  of  their  first  appearance,  we  need  entertain  little  hope 
of  finding  traces  of  a  separation  which  existed,  if  indeed  it 
existed  at  all,  in  a  very  remotely  ancestral  period. 

VI.  The  Development  of  the  Reproductive  Organs 

IN  THE  InSECTA. 

I .  The  Gonads. 

The  following  account  of  the  development  of  the  sexual 
organs  is  based  almost  exclusively  on  Xiphidhmi.  Some 
attention  was  devoted  to  the  study  of  Blatta^  but  this  form 
proved  to  be  so  much  less  satisfactory  and  to  depart  so  little 
from  the  Xiphidmm  type,  that  it  was  abandoned. 

Before  passing  to  a  description  of  the  sexual  organs  and 
their  ducts,  it  will  be  necessary  briefly  to  consider  the  meso¬ 
dermal  somites,  since  the  history  of  the  organs  under  consid¬ 
eration  is  intimately  bound  up  with  the  history  of  the  middle 
germ-layer.  The  mesoderm  of  Xiphidium,  like  that  of  other 
insects,  is  coextensive  with  the  blastopore  and  hence  reaches 
from  the  region  of  the  definitive  mouth  to  the  region  of  the 
definitive  anus.  At  first  a  continuous  cell-layer,  it  soon  splits 
up  into  segmental  masses  as  metamerism  sets  in.  These  are 
further  divided  in  the  median  ventral  line  so  that  each  segment 
has  a  .pair  of  mesoderm  blocks.  Each  of  these  acquires  a 
cavity  and  the  somites  are  established. 

The  appendages  are  from  the  time  of  their  first  appearance 
intimately  connected  with  the  somites,  since  each  of  the  latter 
sends  a  hollow  diverticulum  into  the  appendage  of  the  corre¬ 
sponding  half  of  the  body.  All  the  somites  are  fully  formed 
when  the  embryo  has  reached  Stage  F.  There  are  then  eight¬ 
een  pairs  in  all.  The  most  anterior  pair  occupies  the  deuto¬ 
cerebral  segment  and  sends  hollow  diverticula  into  the 
antennae.  The  walls  of  these  somites  are  much  thinner  than 
those  of  succeeding  pairs  and,  curiously  enough,  persist  much 
longer.  The  pair  in  the  tritocerebral  segment  is  very  small 


WHEELER. 


[VOL.  VIII. 


II4 

and  indistinct  and  disappears  very  early.  Each  of  the  succeed¬ 
ing  segments,  with  the  exception  of  the  eleventh  abdominal 
has  a  distinct  pair  of  somites.  In  the  last  abdominal,  meso¬ 
derm  is  present,  but  I  have  been  unable  to  find  a  trace  of 
a  true  coelomic  cavity.^ 

The  youngest  embryo  in  which  I  was  able  to  detect  repro¬ 
ductive  cells  had  almost  reached  Stage  F.  In  still  earlier 
stages  careful  scrutiny  failed  to  reveal  any  differentiation  of 
the  mesoderm  cells.  These  cells,  it  is  true,  vary  considerably 
in  size  and  appearance  but  I  have  found  it  impossible  to  fix 
on  any  one  set  of  elements  which  might  be  brought  into  con¬ 
nection  with  the  germ-cells  of  older  embryos.  It  is  not, 
therefore,  till  the  somites  are  established  as  distinct  sacs 
that  unmistakable  primitive  germ-cells  make  their  appearance. 
In  frontal  sections  of  embryos  in  Stage  F  (Fig.  52  gd  b  gd3), 
the  primitive  germ-cells  are  seen  to  lie  in  the  inner  wall  of  the 
somite.  They  are  considerably  larger  than  any  of  the  sur¬ 
rounding  mesoderm  cells,  and  much  paler.  The  chromatin  of 
their  nuclei  is  arranged  in  a  more  delicate  skein.  Like  the 
neuroblasts  they  stain  very  deeply  in  picric  acid.  Normally, 
they  occur  only  in  the  first  to  the  sixth  abdominal  segments, 
each  cluster  being  confined  to  the  inner  wall  of  a  somite.  The 
reproductive  organs  of  Xiphidiimi  are  therefore  truly  meta- 
meric  in  their  origin.  There  is  nothing  to  show  that  they 
arise  from  vitellophags  which  have  migrated  into  the  somitic 
wall  ;  nor  can  they  arise  from  the  entoderm,  since  they  are 
differentiated  before  the  entoderm-bands  have  reached  the 
basal  abdominal  segments  in  their  growth  backward  from  the 
oral  and  forward  from  the  anal  formative  centre.  I  conclude, 
therefore,  that  the  primitive  germ-cells  are  enlarged  and  modi¬ 
fied  mesoderm-cells.  In  explanation  of  Fig.  52  it  may  be 
noted  that  the  plane  of  section  is  somewhat  oblique  so  that  it 

1  I  mention  this  because  Graber  (’90)  has  recently  described  a  coelomic  cavity 
in  the  anal  segment  of  Hydrophilus  (Fig.  29,  p.  62).  Cholodkowsky  also  describes 
and  figures  (’91,  Fig.  49  PI.  IV)  such  a  cavity  in  the  eleventh  abdominal  seg¬ 
ment  of  Blatta.  Every  little  slit  in  the  mesoderm  is  not  a  coelomic  cavity,  and 
the  figures  referred  to  show  only  small  spaces  between  the  mesoderm  cells  of 
the  telson.  This  may  have  been  produced  artificially,  for  aught  the  figures  show 
to  the  contrary. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  115 

strikes  only  the  lowermost  germ-cell  of  the  cluster  in  the  third 
abdominal  segment  (s'd3)  and  in  the  fourth  segment  passes 
completely  under  the  cluster.  Even  at  this  lime  certain  meso¬ 
derm-cells,  the  future  epithelial  elements  (ep.)  begin  to  flatten 
out ‘and  apply  themselves  to  the  surfaces  of  the  germ-cells. 

The  exact  relations  of  the  primitive  germ-cells  to  the  walls 
of  the  somite  are  readily  seen  in  transverse  section  (Fig.  53). 
The  inner  face  of  the  triangular  somite  is  applied  to  the  surface 
of  the  yolk  and  besides  giving  rise  to  the  germ-cells  will  ulti¬ 
mately  form  the  splanchnic,  or  visceral  layer.  The  remainder 
of  the  coelomic  wall  is  somatic,  or  parietal,  and  is  converted 
into  fat-body  and  musculature.  The  heart  arises  where  the 
outer  edge  of  the  splanchnic  passes  into  the  somatic  layer. 
In  the  section  figured  the  entoderm  is  still  wanting  on  the  left 
side,  whereas  on  the  right  side  a  single  cell  (en)  of  the  right 
posterior  band  has  already  reached  the  segment.  Similar  ine¬ 
qualities  in  the  rate  of  growth  of  the  entoderm-bands  are  by 
no  means  infrequent. 

In  this  stage  some  of  the  primitive  germ-cells  show  a 
tendency  to  leave  the  wall  of  the  somite  and  to  drop  into  the 
coelomic  cavity.  This  is  distinctly  seen  in  Fig.  53.  These 
cells  sometimes  enlarge  considerably,  become  vacuolated  and 
take  on  the  appearance  of  young  ova.  A  cell  of  this  kind, 
nearly  filling  the  coelomic  cavity,  is  shown  in  Fig.  55.  I  do 
not  believe  that  the  cells  are  loosened  from  the  coelomic  wall 
during  the  process  of  sectioning. 

Although  the  clusters  of  germ-cells  normally  occur  only  in 
the  first  to  the  sixth  abdominal  segments,  in  one  somewhat  older 
embryo  (Stage  G)  a  well  developed  pair  of  clusters  was 
found  in  the  tenth  segment.  One  of  these  is  shown  in 
sagittal  section  in  Fig.  56  It  resembles  the  normal 

clusters  in  every  particular.  The  same  section  shows  the 
diverticulum  of  the  tenth  somite  (m.  d).  In  the  next  section 
laterad  to  the  one  figured,  the  hollow  tip  of  the  diverticulum 
is  seen  to  terminate  in  the  right  appendage  of  the  segment. 
A  similar  relation  of  the  coelomic  diverticula  to  the  append¬ 
ages  obtains  in  all  the  abdominal  segments  in  front  of  the 
tenth. 


WHEELER. 


[VOL.  VIII. 


1 16 

The  primitive  germ-cells,  which  at  first  occupy  only  a  limited 
portion  of  the  splanchnic  wall,  increase  in  number  during  a 
stage  midway  between  F  and  G.  Beautiful  caryokinetic 
figures  may  then  be  found  in  some  specimens  —  showing  that 
the  primitive  germ-cells  themselves  proliferate.  New  germ- 
cells  may  be  added  to  the  clusters  by  a  differentiation  of 
elements  in  the  splanchnic  wall  but  I  have  seen  nothing  to 
convince  me  that  this  occurs.  The  epithelial  cells  become 
much  flattened  and  stain  more  deeply  so  that  they  stand  out 
distinctly  among  the  pale  rounded  germ-cells  which  they 
invest.  The  inner  wall  of  the  somite  soon  becomes  too 
small  to  contain  all  the  rapidly  accumulating  cells  and  is 
forced  to  send  out  a  solid  diverticulum.  This  is  directed 
anteriorly,  and  in  a  little  later  stage  fuses  with  the  wall  of  the 
antecedent  somite.  This  fusion  is  probably  preceded  by  the 
shortening  of  the  embryo  which  takes  place  during  a  stage 
immediately  succeeding  Stage  F.  The  result  of  the  fusion  is 
the  formation  of  a  continuous  strand  of  germ-cells  with  their 
accompanying  epithelial  cells.  For  some  time  the  typical  hexa- 
metameric  arrangement  is  still  visible  in  the  strand,  but  later 
the  whole  mass  shortens  very  decidedly  to  form  the  definitive 
ovary  and  testis  and  all  traces  of  metamerism  are  lost.  In  the 
present  paper  I  shall  not  follow  the  development  of  these 
organs  further,  but  will  pass  on  to  a  description  of  the  sexual 
ducts.  This  will  enable  me  to  supplement  the  recent  work  of 
Heymons  (’9i)  who  has  given  us  an  extended  and  valuable 
account  of  the  development  of  the  sexual  organs  in  Blatta^  but 
has  contributed  only  a  few  observations  on  the  development  of 
the  ducts. 


2.  The  Male  Ducts. 

The  sexual  ducts  like  the  germ-cells  are  modified  portions 
of  the  mesodermal  somites.  While  considering  the  excep¬ 
tional  embryo  in  Fig.  56,  attention  was  directed  to  the 
diverticulum  ((in.  d)  of  the  tenth  abdominal  somite.  This 
diverticulum,  which  is  quite  normal,  is  destined  to  form  the 
terminal  portion  of  the  deferent  duct  (spermaduct)  and  the 
seminal  vesicle  of  the  adult  insect.  At  the  base  of  the  divert- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  117 

iculum  a  constriction  is  formed  which  converts  the  proximal 
portion  into  a  thin  cord  but  leaves  the  distal  end  expanded  as 
a  hollow  sac,  which  I  shall  call  the  terminal  ampulla.  The 
remainder  of  the  deferent  duct  —  viz.  the  portion  extending  from 
the  sexual  gland  in  the  sixth  to  the  anterior  end  of  the  tenth 
segment  is  formed  by  a  cord-like  thickening  in  the  splanchnic 
wall  of  the  three  intervening  somites.  Anteriorly  the  cells 
of  the  duct  pass  into  the  epithelium  enveloping  the  germ-cells. 
There  is  no  lumen  in  the  duct  proper  except  towards  its  end 
where  it  widens  into  the  terminal  ampulla.  Thus  only  the  ap¬ 
pendage  diverticula  of  the  tenth  segment  go  to  form  the  ends 
of  the  ducts  ;  in  all  the  other  abdominal  segments  the  diverti¬ 
cula  break  down  and  disappear,  together  with  their  respective 
appendages,  before  the  embryo  reaches  Stage  H.  In  the 
thoracic,  oral  and  antennary  segments,  however,  the  diverti¬ 
cula  are  converted  into  the  muscles  of  the  persistent  ap¬ 
pendages. 

The  further  history  of  the  male  ducts  is  readily  followed  in 
partially  stained  embryos  mounted  in  toto.  Sex  is  determined, 
so  far  as  I  have  been  able  to  make  out,  during  or  soon  after 
katatrepsis,  at  which  time  the  appendages  of  the  second  to 
the  seventh  abdominal  segments  disappear.  Fig.  42  represents 
the  caudal  end  of  an  embryo  just  after  katatrepsis  (Stage  J). 
Appendages  still  persist  on  the  eighth  to  eleventh  segments 
while  the  pleuropodia,  not  seen  in  the  figure,  have  begun  to 
degenerate.  The  testes  {tsi)  and  the  spermaducts  {m.  d.)  are 
represented  in  blue.  The  former  have  shortened  considerably 
and  moved  caudad  so  that  they  now  lie  in  the  sixth  and  seventh 
segments.  The  long  terminal  threads  run  forwards  from  the 
anterior  ends  of  the  testes,  while  the  spermaducts  run  back¬ 
wards  and  end  in  the  terminal  ampullae  {ta.  m.)  which  still  fit 
into  the  cavities  of  the  tenth  pair  of  abdominal  appendages 
{ap.  10).  A  section  through  these  appendages  is  seen  in  Fig. 
57,  showing  very  clearly  the  connection  of  the  ampullae  {ta.  m) 
with  the  ducts  {7n.  d.).  In  front  of  this  section  the  ducts  have 
no  distinct  lumen. 

In  Fig.  43,  taken  from  a  slightly  older  embryo,  the  append¬ 
ages  of  the  eighth  segment  have  completely  disappeared, 


ii8 


WHEELER, 


[VOL.  VIII. 


while  those  of  the  tenth  have  grown  smaller  and  approached 
the  median  ventral  line.  They  have,  in  fact,  grown  too  small 
to  contain  the  ampullae  which  are  drawn  away  from  them  and 
lie  between  the  ninth  and  tenth  segments  a  little  in  front  of 
the  abortive  appendages.  At  the  same  time  the  inner  faces  of 
the  ampullae  have  become  flattened  and  applied  to  each  other 
in  the  median  line.  Each  of  these  sacs  has  a  pointed  tip 
directed  caudad.  The  more  arcuate  course  described  by  the 
ducts  in  this  stage  is  undoubtedly  due  to  the  mutual  approxi¬ 
mation  of  their  terminal  ampullae.  The  appendages  of  the 
eleventh  segment,  the  cerci  {cc.  [<^/“]),  which  in  the  pre¬ 
ceding  stage  were  rounded  like  the  other  abdominal  append¬ 
ages,  have  become  oval. 

A  more  advanced  embryo  is  represented  in  Fig.  44  (some¬ 
what  younger  than  Stage  K).  The  appendages  of  the  tenth 
segment  {ap^^)  have  almost  completely  disappeared.  Those 
of  the  ninth  segment,  the  future  styli  {ap9)  have  lengthened 
and  now  point  outwards  and  forwards.  The  cerci  have  grown 
more  pointed.  The  terminal  ampullae  lie  completely  in  the  ninth 
segment,  having  shifted  their  position  headwards.  The  move¬ 
ment  takes  place  in  such  a  way  that  what  were  the  posterior 
faces  of  the  ampullae  in  the  younger  stage  (Fig.  43)  are  applied 
to  each  other,  while  the  pointed  tips  are  directed  forwards. 
The  ducts  are  thereby  rendered  still  more  arcuate  towards 
their  terminations.  An  intermediate  stage  in  this  singular 
movement  is  shown  in  Fig.  45,  where  only  small  portions  of 
the  posterior  faces  of  the  ampullae  are  as  yet  applied  to  each 
other.  Comparison  of  Figs.  43,  44  and  45  shows  that  the 
pointed  tips  remain  united  and  move  forward  while  the  sur¬ 
faces  of  mutual  contact  are  being  shifted.  Finally  in  Fig.  46, 
which  represents  the  abdominal  end  of  an  embryo  ready  to 
hatch,  we  see  that  the  terminal  ampullae  have  increased  con¬ 
siderably  in  size  at  the  expense  of  the  thickness  of  their  walls. 
They  have  also  lengthened,  and  brought  still  more  of  their 
surfaces  in  contact  in  the  median  line.  The  pointed  tips  of 
the  ampullae  extend  into  the  eighth  segment.  It  may  also  be 
noted  that  the  points  where  the  spermaducts  meet  the  ampullae 
have  moved  forward.  The  appendages  of  the  tenth  segment 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


II9 

have  long  since  disappeared  and  the  pleuropodia  have  lost 
their  organic  connection  with  the  embryo,  so  that  only  two 
pairs  of  abdominal  appendages  persist,  the  stylets  {st.  \ap  9] ) 
and  the  cerci  {cc.  [<^/^^]),  both  provided  with  setae.  The 
pointed  fusiform  cerci  are  now  folded  back  so  as  to  bring 
their  insertions  on  the  anal  segment  into  view. 

Beyond  this  stage  the  development  of  the  male  ducts  was 
not  followed  in  Xiphidium  ensifertmt,  but  several  larval  stages 
of  an  allied  species,  X.  fasciatum,  were  studied  for  the  purpose 
of  connecting  the  embryonic  with  the  adult  condition. 

It  will  be  noticed  that  in  Xiphidiu^n  ensiferum  there  exists 
at  the  time  of  hatching  no  external  opening  to  the  sperma- 
ducts;  the  ampullae  are  completely  closed  sacs  applied  to 
the  ventral  hypodermis  of  the  ninth  abdominal  segment,  and 
the  ducts  connecting  them  with  the  testes  have  no  lumen. 
In  the  X.  fasciatitm  larva  10  mm.  long  the  ejaculatory  duct 
has  made  its  appearance  as  an  unpaired  invagination  of  the 
hypodermis  in  the  median  line  between  the  ninth  and  tenth 
segments.  Fig.  47  shows  the  sexual  organs  of  such  a  larva 
seen  from  within,  the  ventral  scutes  of  the  tenth  and  anal  seg¬ 
ments  having  been  entirely  removed.  The  prominent  terminal 
ampullae,  which  become  the  seminal  vesicles  of  the  adult,  are 
considerably  enlarged  and  their  walls  have  increased  in  thick¬ 
ness.  The  short  spermaducts,  now  provided  with  a  small 
lumen,  run  from  the  under  surface  of  the  sacs  to  the  prom¬ 
inent  testes.  Only  the  outer  opening  of  the  invagination 
which 'is  to  form  the  ejaculatory  duct  is  seen  at  m.o.  It  runs 
forward  as  a  flattened  chitin-lined  depression  beneath  the 
seminal  vesicles.  Sagittal  sections  show  that  there  is  as  yet 
no  communication  between  the  lumina  of  the  mesodermal  and 
ectodermal  portions  ;  it  is  not  till  a  later  stage  that  such  a 
communication  is  established. 

3.  The  Female  Ducts. 

The  oviducts,  like  the  vasa  deferentia,  are  derived  from  a 
pair  of  coelomic  appendage-diverticula,  but  in  the  female  the 
diverticula  belong  to  the  seventh  abdominal  segment.  The 
diverticula  of  the  female  embryo  also  become  constricted 


I  20 


WHEELER. 


[VOL.  VIII. 


proximally  and  end  in  terminal  ampullm,  which  are  from 
the  first  somewhat  smaller  and  more  elongate  than  the  homo- 
dynamous  structures  of  the  male.  The  appendages  to  which 
the  ampullae  belong  are  also  less  prominent  than  the  tenth 
pair  of  appendages  in  the  male.  Examination  of  several 
series  of  cross-sections  from  embryos  in  Stage  J  —  this  being 
the  stage  in  which  the  sexes  differentiate  —  reveals  the  curious 
fact  that  in  the  female,  besides  the  pair  of  ampullae  in  the  sev¬ 
enth,  a  pair  is  also  retained  in  the  tenth  segment.  Figs.  58  and 
59  represent  two  sections  taken  from  such  an  embryo  —  the 
former  passing  through  the  tenth,  the  latter  through  the  seventh 
abdominal  segment.  In  Fig.  58,  the  two  terminal  ampullae 
{ta.  m.),  and  small  portions  of  the  ducts  leading  to  them,  are 
still  preserved,  but  the  cells  and  nuclei,  especially  in  the  ducts, 
are  being  broken  down.  The  ampullae  soon  share  the  same 
fate.  In  the  seventh  segment  (Fig.  59, /*.</.)  the  cavity  of  the 
diverticulum  still  opens  into  the  coelomic  cavity  of  the  same 
segment  (coe).  Its  distal  ampullar  end  is  applied  to  the  ecto¬ 
derm  where  it  bulges  out  to  form  the  small  seventh  abdominal 
appendage  {ap7).  The  condition  of  the  diverticulum  after  the 
constriction  of  its  proximal  portion  is  shown  in  Fig.  60,  taken 
from  a  somewhat  more  advanced  embryo.  In  this  figure,  the 
connection  of  the  oviduct  with  the  posterior  end  of  the  young 
ovary  (ov.)  is  distinctly  seen.  The  cells  of  the  duct  pass  over 
into  the  epithelial  cells  of  the  ovary,  just  as  the  cells  of  the 
spermaducts  become  continuous  with  the  testicular  epithelium. 

We  may  now  turn  to  surface  views  of  the  female  reproduc¬ 
tive  organs.  The  specimen  represented  in  Fig.  48  is  in  the 
same  stage  as  the  male  embryo  represented  in  Fig.  42.  Five 
consecutive  pairs  of  abdominal  appendages  are  still  present 
{apy ,  Of  these,  the  ninth  and  eleventh  pairs  are  very 

prominent,  while  the  tenth  pair  has  grown  very  small.  The 
ovary  (ov.)  extends  back  to  the  seventh  segment  where  it  joins 
the  oviduct.  This  ends  in  the  terminal  ampulla,  which  lies  near 
the  posterior  edge  of  the  segment  in  the  seventh  abdominal 
appendage.  The  terminal  ampullae  of  the  tenth  segment  have 
not  yet  disappeared.  They  are  represented  in  blue  because  I 
regard  them  as  the  homologues  of  the  persistent  male  ampullae. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY. 


I2I 


In  a  more  advanced  embryo  (Fig.  49,  Stage  K)  the  male 
ampullae  have  disappeared  completely,  and  the  tenth  pair  of 
appendages,  while  growing  smaller,  have  moved  up  to  the  inner 
posterior  insertions  of  the  ninth  pair.  The  ampullae  have  in¬ 
creased  in  size  and  have  come  to  lie  at  right  angles  to  the  lon¬ 
gitudinal  axis  of  the  embryo.  This  causes  the  oviducts  to 
describe  an  arc.  It  is  thus  seen  that  the  movement  of  the 
female  ampullae  is  essentially  the  same  as  that  of  the  male,  but 
considerably  weaker.  Traces  of  appendages  on  the  seventh 
segment  are  still  apparent. 

From  this  stage  we  may  pass  to  a  brief  consideration  of  the 
female  embryo  ready  to  hatch  (Fig.  50).  The  ovaries  {ov}j 
have  now  assumed  their  definitive  characters.  Although  the 
pointed  and  flattened  ampullae  have  approached  the  median 
ventral  line,  they  are  still  separated  by  a  wide  space.  Even  in 
this  advanced  stage  slight  thickenings  of  the  integument  over 
the  posterior  edges  of  the  ampullae  may  be  taken  to  represent 
the  remains  of  the  seventh  pair  of  abdominal  appendages.  The 
appendages  of  the  tenth  segment  appear  to  have  joined  the 
inner  bases  of  the  ninth  pair.  I  must  say,  however,  that  my 
observations  on  this  pair  of  appendages  are  unsatisfactory, 
notwithstanding  I  have  taken  considerable  pains  to  follow  their 
history.  The  appendages  of  the  eighth  and  ninth  segments 
undoubtedly  form  the  two  anterior  pairs  of  gonapophyses. 
The  third  pair  has  been  described  by  Dewitz  (’75)  and  others  as 
arising  from  the  inner  bases  of  the  second  pair  and  is  therefore 
supposed  to  belong  to  the  ninth  segment.  I  believe,  however, 
that  the  tenth  pair  of  embryonic  appendages  persists  and  moves 
forward  to  join  the  ninth  pair,  whence  they  grow  out  during 
early  larval  life  as  the  third  pair  of  gonapophyses.  In  the 
embryo  the  line  separating  the  ninth  and  tenth  segments  is 
certainly  very  vague,  especially  on  the  ventral  surface,  so  that 
the  possibility  of  a  fusion  of  the  two  pairs  of  appendages  is  by 
no  means  precluded.  That  this  fusion  should  occur  is  certainly 
no  more  remarkable  than  the  migration  of  the  male  ampullae 
from  the  tenth  to  the  ninth  segment.  Both  of  these  forward 
movements  may  be  in  some  way  connected  with  the  forward 
migration  and  fusion  of  the  ganglia  belonging  to  the  eighth, 
ninth,  and  tenth  segments  {cf.  Figs.  42—46,  ag^-^. 


122 


WHEELER. 


[VOL.  VIII. 


The  female  larva,  like  the  male,  has  no  external  orifice  to 
the  sexual  organs  at  the  time  of  hatching.  It  is  even  more 
backward  than  the  male,  inasmuch  as  the  terminal  ampullae  of 
‘the  oviducts  have  not  yet  met  in  the  median  ventral  line.  The 
first  traces  of  a  vagina  were  found  in  Xiphidiuni  fasciatum 
larvae  about  lo  mm.  long  (Fig.  51).  Here  the  terminal  am¬ 
pullae  meet,  but  the  surfaces  of  mutual  contact  are  limited  to 
the  pointed  tips.  The  vagina  ivg)  is  a  short  and  broad  in¬ 
vagination  of  the  hypodermis  between  the  seventh  and  eighth 
segments.  Its  tip  extends  to  the  juncture  of  the  terminal 
ampullae.  In  a  somewhat  later  stage  the  ampullae  open  into 
each  other  and  into  the  vagina.  The  three  pairs  of  gonapo- 
physes  (op^-opz)  are  already  assuming  their  definitive  charac¬ 
ters. 


For  an  excellent  resume  of  the  little  work  that  has  been  done 
on  the  embryonic  development  of  the  sexual  organs  in  insects 
I  would  refer  the  reader  to  Heymons’  recent  paper  (’9i).  Here 
I  shall  consider  only  three  contributions,  —  two  of  Heymons’ 
(’90  and  ’9i)  which  treat  mainly  of  the  sexual  glands,  and  Nus- 
baum’s  paper  on  the  development  of  the  ducts  (’84). 

Although  the  results  of  my  study  of  Xiphidiiim,  so  far  as 
they  go,  agree  in  many  respects  with  Heymons’  account  of 
Blatta,  several  not  unimportant  differences  must  be  pointed 
out.  The  first  difference  relates  to  the  stage  in  which  the 
germ-cells  make  their  appearance.  Heymons  (’9l)  claims  that 
he  can  detect  them  before  the  somites  are  established,  at  a  time, 
in  fact,  when  the  abdominal  region  of  the  embryo  is  still  un¬ 
segmented.  This  would  correspond  to  a  stage  in  Xiphiditim 
midway  between  B  and  C.  In  Blatta  certain  mesoderm-cells 
at  this  time  enlarge  and  assume  a  clear  and  succulent  appear¬ 
ance.  There  is  apparently  no  definite  relation  between  the 
position  of  these  modified  cells  and  the  future  segments,  and 
even  in  a  later  stage  when  they  become  integral  portions  of 
the  somite-wall,  they  are  quite  irregular  in  their  distribution. 
Heymons  regards  them  as  largely  dissepimental  in  position. 
In  Xiphiditim,  which  is,  on  the  whole,  a  far  more  favorable 
form  for  the  study  of  the  sexual  organs  than  Blatta,  I  was 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY,  123 

unable  to  detect  the  presence  of  germ-cells  till  the  somites 
were  established  (Stage  a  little*  younger  than  F).  At  this  time 
they  formed  strictly  metameric  cell-clusters  each  of  which  was 
confined  to  the  median  portion  of  the  splanchnic  wall  of  its 
respective  somite.  These  cells  rarely,  if  ever,  strayed  into  the 
dissepimental  region  during  this  stage.  It  is,  of  course,  con¬ 
ceivable,  that  Xiphidiinn  and  Blatta  may  differ  very  consider¬ 
ably  in  respect  to  the  point  under  consideration,  but  I  suspect, 
nevertheless,  that  Heymons  has  mistaken  the  young  vitellophags 
for  sexual-cells,  notwithstanding  his  assertion  to  the  contrary. 
At  any  rate,  to  be  complete,  his  figures  should  show  the  vitel¬ 
lophags,  which  are  undoubtedly  present  in  the  stages  he 
studied  and  which  occupy  the  very  location  of  his  “sexual- 
cells”  in  his  Figs.  2  and  3. 

Another  point  on  which  we  differ  is  the  distribution  of  the 
germ-cells.  According  to  Heymons  they  occur  in  Blatta  in 
the  second  to  seventh  abdominal  segments,  whereas  I  find 
them  in  Xiphidium  in  the  first  to  sixth.  Heymons  says 
emphatically:  “  Im  ersten  Abdominalsegment  treten  niemals 
Genitalzellen  auf.”  But  this  is  certainly  an  error,  for 
in  several  Blatta  embryos  I  find  unmistakable  germ-cells 
forming  a  pair  of  isolated  clusters  in  the  first  abdominal 
segment.  Here  they  also  persist  till  a  comparatively  late 
stage  when  they  are  drawn  into  the  second  segment  during 
the  contraction  of  the  sexual  Anlage.  The  peculiarly  modified 
pleuropodia  in  Blatta  form  so  efficient  a  means  for  determin- 
ing  the  exact  position  of  the  first  abdominal  segment  and  its 
somite  in  series  of  sections  both  longitudinal  and  transverse, 
that  I  feel  confident  of  not  being  mistaken  in  this  matter.  I 
admit,  however,  that  fewer  germ-cells  occur  in  the  first  than  in 
the  succeeding  abdominal  segments.  According  to  Heymons 
comparatively  few  germ-cells  occur  in  the  seventh  pair  of 
abdominal  somites.  In  these  I  have  never  seen  traces  of 
germ-cells  in  Xiphidium  but  I  cannot,  of  course,  assert  that 
they  never  occur,  especially  as  I  have  shown  that  germ-cells 
may  be  found  even  as  far  back  as  the  tenth  segment.  It  is 
interesting  to  note  that  Heymons,  too,  found  germ-cells  in  some 
of  the  posterior  abdominal  segments  in  Blatta. 


124 


WHEELER. 


[VOL.  VIII. 


In  his  first  paper  (’90)  Heymons  made  the  following  state¬ 
ment  in  regard  to  the  genital  ducts  :  “Von  besonderer 
Wichtigkeit  scheint  mir  nun  die  Thatsache  zu  sein,  dass 
beim  Mannchen  der  urspriinglich  angelegte  Ausfiihrungs- 
gang  nicht  in  seiner  ganzen  Lange  zum  Vas  deferens  wird, 
sondern  dass  sich  sein  distaler  Abschnitt  spater  wieder  zu- 
riickbildet,  ohne  je  functioniert  zu  haben.  Der  wirklich  als 
Ausfiihrungsgang  dienende  Endtheil  des  Vas  deferens,  welcher 
sich  mit  dem  ectodermalen  Ductus  ejaculatorius  verbindet,  ent- 
steht  erst  nachtraglich  an  dem  urspriinglich  angelegten  Aus- 
fiihrungsgang.  Beim  Weibchen  dagegen  bildet  sich  der  ganze 
primitive  Ausfuhrungsgang  zum  Oviduct  aus.” 

This  is  the  very  opposite  of  what  I  have  found :  in  Xiphidittm 
it  is  the  male  duct  which  at  first  occurs  in  both  sexes  in  the 
tenth  abdominal  segment  —  whereas  in  the  female  the  oviducts 
are  an  independent  formation,  the  original  male  duct  being 
soon  broken  down.  In  the  female  both  pairs  of  ducts  are 
established  simultaneously  since  they  are  both  coelomic 
diverticula.^ 

In  his  more  recent  paper  Heymons  (’9i)  describes  the  genital 
ducts  as  terminating  at  the  posterior  edge  of  the  seventh 
abdominal  segment.  As  he  mentions  this  fact  before  he 
comes  to  a  description  of  the  embryo  with  determinate  sex, 
I  assume  that  he  regards  these  ducts  as  common  to  both  sexes. 
What  he  saw  was  without  doubt  the  pair  of  oviducts,  not  the 
deferent  ducts.  From  personal  observation  I  can  state  that 
the  male  ducts  of  Blatta  end  at  first  in  terminal  ampullae 
enclosed  by  the  appendages  of  the  tenth  abdominal  segment 
just  as  in  XipJiidium,  whereas  the  female  ducts  terminate 
in  much  flattened  ampullae  in  the  seventh  segment.  Whether 
or  not  a  rudimental  male  duct  is  present  in  the  tenth  segment 
of  the  female  Blatta  embryo  I  have  been  unable  to  decide. 
Perhaps  Heymons  found  something  of  this  kind  and  while 
confounding  the  sex  of  the  embryos  he  studied,  was  led  to 
make  the  above  quoted  remark. 

1  For  the  sake  of  greater  exactness,  I  may  state  that  the  anterior  pair  is,  perhaps, 
formed  a  little  sooner  than  the  posterior  pair,  since  the  somites  develop  from 
before  backwards. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  125 


Of  the  few  observations  which  have  been  made  on  the 
development  of  the  genital  ducts  in  insects,  Nusbaum’s  (’84) 
are  the  most  important.  Their  agreement  with  Palmen’s 
anatomical  researches  on  Ephemerids  (’84)  has  been  regarded  as 
sufficient  warrant  of  their  accuracy.  Nusbaum  studied  the 
developing  ducts  in  Mallophaga,  Pediculidae,  Blattidae,  and 
Culicidae  and  came  to  the  conclusion  that  the  testes  and  defer¬ 
ent  ducts  in  the  male  and  the  ovaries  and  oviducts  in  the 
female  are  mesodermal,  while  the  seminal  vesicles,  ejaculatory 
duct  and  accessory  glands  in  the  male,  and  the  uterus,  vagina 
and  accessory  glands  in  the  female  are  ectodermal.  He 
therefore  draws  the  line  between  mesodermal  and  ectodermal 
portions  at  the  juncture  of  the  seminal  vesicles  and  deferent 
ducts  in  the  male  and  at  the  juncture  of  the  oviducts  and 
uterus  in  the  female.  This  is  at  variance  with  my  results, 
since  I  have  found  that  the  seminal  vesicles  and  uterus  are 
mesodermal.  These  structures  are  described  by  Nusbaum  as 
if  he  had  traced  their  derivation  from  the  ectoderm  step  by 
step.  Yet  he  seems  not  to  have  studied  embryos  but  only 
larvae,  and  it  is  during  embryonic  life  that  the  uterus  and 
seminal  vesicles  are  formed.^  Furthermore  his  investigations 
were  carried  on  without  the  aid  of  sections.  The  differentia¬ 
tion  of  the  uterus  and  seminal  vesicles  from  the  ectoderm  is 
far  from  satisfactorily  shown  in  his  figures.  I  cannot  therefore 
regard  Nusbaum’s  work  as  contributing  any  evidence  in  favor 
of  Palmen’s  view.  Palmen  concluded  from  a  careful  study  of 
the  Eph'emeridae  that  the  genital  ducts  of  insects  originally 


1  This  is  Nusbaum’s  description  (’84,  p.  40):  “  Auf  der  Bauchseite  des  vierten, 
von  hinten  an,  Abdominalsegmentes  entstehen  zwei  paarige  Hautepithelverdick- 
ungen  die  sich  einander  nahern  um  sich  dann  zu  einem  hufeisenformigen  unpaaren 
Kdrper  zu  vereinigen.  Bevor  aber  noch  die  Vereinigung  zu  Stande  kommt,  losen 
sich  diese  Keime  von  der  Haut  ab  und  verwachsen,  wie  gesagt,  mit  den  Enden 
der  noch  soliden  Vasa  deferentia.  ...  In  dem  vorderen  Theile  des  soliden  huf* 
eisenfdrmigen  Keimes  entstehen  zwei  vordere  geschlossene  Hohlen;  der  mittlere 
Theil  bleibt  noch  weiter  solid,  der  hintere  verlangert  sich  in  zwei  seitliche  solide 
Auswiichse.  Die  zwei  vorderen  Hohlen  verlangern  sich  nach  vorn  und  differ- 
enziren  sich  in  die  zwei  Vesiculae  seminales  (innere  Schlauche)  des  definitiven 
birnfdrmigen  Kbrpers.  Mit  denselben  communiciren  die  sich  aushohlenden  Vasa 
deferentia.”  An  essentially  similar  description  is  given  by  Nusbaum  for  the 
female  (p.  41). 


126 


WHEELER, 


[VOL.  VIII. 


had  paired  openings  on  the  surface  of  the  body.  This  view, 
which  I  fully  endorse,  has  a  good  basis  of  anatomical  data  ; 
but  Nusbaum  has  not  shown  that  there  is  a  double  opening 
to  the  sexual  organs  or  even  a  distinctly  paired  Anlage  to  the 
ectodermal  portion  of  the  sexual  apparatus.  According  to 
his  own  figures  the  vagina  and  ejaculatory  duct  are  unpaired 
from  the  first  ;  the  structures  on  which  he  laid  stress  as  being 
paired  ectodermal  portions  were  nothing  more  nor  less  than  the 
unmodified  terminations  of  the  mesodermal  ducts  —  the  ter¬ 
minal  ampullae. 

4.  General  Considerations. 

The  foregoing  account  of  the  development  of  the  sexual 
organs  differs  sufficiently  from  the  accounts  of  other  authors 
to  justify  a  brief  consideration  of  some  general  questions. 

First  in  regard  to  the  germ-cells.  These  arise  as  six  meta- 
meric  pairs  of  clusters  in  the  splanchnic  walls  of  the  meso¬ 
dermal  somites.  Since  the  single  layer  of  cells  forming  the 
walls  of  each  somite  corresponds  to  the  peritoneal  epithelium 
of  Annelids,  Heymons’  conclusion  that  the  germ-cells  of  insects 
arise  in  essentially  the  same  manner  as  the  germ-cells  of 
Annelids,  is  certainly  well-founded.  In  both  groups  the  germ- 
cells  are  local  proliferations  of  the  epithelium  lining  the  body 
cavity.  In  Annelids  the  germ-cells  lose  their  connection  with 
the  peritoneum  and  drop  into  the  body  cavity  where  they 
undergo  maturation.  I  have  called  attention  to  a  similar 
process  in  the  Xiphidium  embryo.  Whether  these  germ-cells 
disintegrate  or  again  attach  themselves  to  the  wall  and  become 
invested  with  epithelial  cells,  I  must  leave  undecided.  I  am 
inclined  to  adopt  the  latter  alternative,  since  I  have  found  no 
traces  of  germ-cells  in  the  coelomic  cavities  in  stages  but  little 
older  than  the  one  figured.  (Fig.  53.)  Heymons  has  observed 
in  Blatta  a  similar  migration  of  the  germ-cells  into  the 
coelomic  cavity. 

I  have  alluded  to  the  fact  that  Xiphidium  exhibits  more 
pronounced  metamerism  in  the  early  arrangement  of  the- 
germ-cells  than  Blatta.  Strictly  speaking  the  germ-cells  in 
the  form  studied  by  Heymons  are  not  at  all  metameric  since 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  127 


they  arise,  if  his  account  is  correct,  before  metameres  are 
established.  It  is  only  on  the  supposition  that  the  germ-cells 
of  Blatta  are  precociously  segregated  that  their  method  of 
origin  can  be  satisfactorily  compared  with  the  conditions  seen 
in  Annelids,  for  in  this  group  the  germ-cells  are  not  differ¬ 
entiated —  so  far  as  I  am  aware  —  until  after  the  somites  have 
reached  a  considerable  degree  of  development.  Providing, 
therefore,  that  I  have  not  overlooked  the  germ-cells  in  precoelo- 
mic  stages,  Xiphiditmi  must  be  regarded  as  presenting  more 
primitive  conditions  than  Blatta. 

In  Xiphidhmt  and  Blatta  six,  and  therefore  more  than  half 
the  total  number  of  abdominal  segments,  produce  germ-cells.^ 
In  one  case  I  found  well-developed  clusters  in  the  tenth 
segment,  so  that  if  we  omit  the  eleventh  or  telson-segment, 
which  is  rudimental  and  hence  cannot  be  expected  to  produce 
germ-cells,  and  if,  moreover,  Heymons  is  correct  in  stating  that 
reproductive  elements  occur  in  the  seventh,  only  two  abdominal 
segments  fail  to  produce  germ-cells  !  This  consideration  lends 
support  to  Heymons’  suggestion  that  “  iirspriinglich  die  Sexual- 
zellen  auch  in  den  hinteren  Segmenten  des  Abdomens  noch  in 
derselben  typischen  Weise  auftraten.”  The  resemblance  of 
the  insect-embryo  to  Annelids  in  which  a  great  number 
of  consecutive  segments  produce  ova  and  spermatozoa,  is  very 
obvious.  The  high  development  of  the  appendages  and  their 
musculature  in  the  thoracic  and  oral  segments  of  insects  per¬ 
haps  sufficiently  accounts  for  the  complete  elimination  of  the 
germ-cell  clusters  in  these  regions.  It  may  also  be  noted  that 
they  are  normally  absent  in  the  abdomen  in  the  very  segments 
which  longest  retain  traces  of  quondam  ambulatory  append¬ 
ages — ^viz.  the  eighth  to  the  eleventh. 

The  indications  of  metamerism  which  are  so  transitory  in 
the  sexual  Anlage  of  the  Orthoptera  would  appear  to  be  retained 
throughout  life  in  some  of  the  Thysanura.  In  lapyx,  according 
to  Grassi  (’89),  the  arrangement  of  the  egg-tubes  is  “  nettement 
mdtamerique,”  and  his  Fig.  44,  PI.  IV,  represents  in  either 
ovary  seven  egg-tubes,  occurring  in  consecutive  abdominal  seg- 

1  In  a  single  instance  (Fig.  55,  PI.  VI)  what  I  took  to  be  a  sexual  cell  was 
found  in  one  of  the  coelomic  cavities  of  the  metathoracic  segment ! 


128 


WHEELER. 


[VOL.  VIII. 


ments,  beginning  with  the  first.  The  solid  testes  show  no 
traces  of  metamerism.  In  the  young  Lepisma,  according  to 
the  same  authority,  the  egg-tubes  are  also  segmental,  there  be¬ 
ing  five  in  either  ovary,  a  pair  in  each  of  the  five  basal  abdominal 
segments.  In  the  adult  this  character  is  no  longer  noticeable. 
In  certain  species  of  Lepismina^  there  are  six  sacs  in  either 

testicle,  united  in  pairs  on  either  side.  These  also  lie  in  the 

• 

basal  abdominal  segments.  “  L’ organisation  segmentaire  des 
ovaires  chez  lapyx  se  repete  chez  Machilis.  Cette  repetition 
dans  des  formes  tres  eloignes  I’une  de  I’autre  comme  le  sont 
precisement  lapyx  et  Machilis,  tend  a  donner  au  fait  une 
serieuse  valeur  morphologique.  Les  tubes  ovariques  de  Machilis 
sont  au  nombre  de  sept  de  chaque  cote.”  In  the  latter  form 
Oudemans  (’87)  also  figures  seven  egg-tubes  strung  along  the 
oviduct,  but  without  a  clearly  marked  metameric  arrangement. 
In  the  male  there  are  three  pairs  of  testicular  sacs. 

In  all  these  Thysanura  the  female,  as  might  be  expected,  ad¬ 
heres  more  tenaciously  than  the  male  to  the  metameric  scheme. 
It  will  also  be  observed  that  the  number  of  egg-tubes  (five  to 
seven  pairs)  is  about  the  same  as  the  number  of  germ-cell  clus¬ 
ters  in  embryo  Orthoptera.  The  position  of  the  organs  is  also 
identical,  viz.  in  the  first  to  seventh  abdominal  segments.  We 
might  conclude,  therefore,  that  the  sexual  organs  of  the  higher 
Thysanura  represented  an  embryonic  or  arrested  condition. 

A  difficulty  is  encountered,  however,  when  we  stop  to  ask 
the  question:  Is  the  individual  egg-tube  in  such  a  form  as  Ma¬ 
chilis  homologous  with  an  individual  egg-tube  in  Blatta  or  any 
other  Pterygote }  So  far  as  structure  is  concerned,  this  would 
appear  to  be  the  case.  We  should  also  say  that  each  egg-tube 
of  lapyx  or  Machilis  was  a  metameric  unit.  But  the  lowest 
number  of  egg-tubes  in  the  Blatta  ovary  is  sixteen,  and  as  this 
is  more  than  double  the  number  of  metameres  which  contribute 
germ-cells  in  the  embryo,  the  egg-tube  in  this  form  cannot  be 
regarded  as  a  metameric  unit.  We  must  conclude,  therefore, 
either  that  the  individual  egg-tubes  are  not  homodynamous  in 
the  Pterygota  and  Apterygota,  or  that  the  ovaries  in  lapyx, 
Machilis,  etc.,  are  not  primitively  metameric.  The  possibility 
of  there  being  an  acquired  metamerism,  or  pseudometamerism 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  129 


in  these  cases  is  suggested  by  Grassi :  “  Cette  disposition  qui 
est  nettement  metamerique  preserve  Tovaire  du  danger  d’etre 
deteriore  de  quelque  maniere  que  ce  soit.  Le  danger  existe 
particulierement  quand  les  oeufs  sont  pres  d’arriver  a  maturite 
et  provient  de  ce  que,  chez  lapyx,  la  differentiation  segmentaire 
de  la  musculature  et  de  la  cuticule  est  avancee  au  point  que  les 
metameres  ont  acquis  une  grande  independance  de  mouvement. 
Cette  independance  est  beaucoup  moindre  chez  Campodea  et 
c’est  pour  cela  que  cet  insecte  n’ off  re  pas  la  disposition  indique 
plus  haut.”  If  this  be  an  adequate  explanation,  the  resemblance 
of  the  sexual  organs  in  the  Thysanura  to  those  in  the  Orthoptera 
is  due  to  secondary  causes.  At  all  events,  this  question  must 
remain  open  till  Thysanuran  embryos  can  be  studied. 

The  metameric  mesodermal  origin  of  the  germ-cells  in 
embryo  Orthoptera  is  too  much  like  the  origin  of  the  germ- 
cells  in  Annelids  to  be  considered  as  secondary  and  I  fully 
agree  with  Heymons  (’9l),  and  Korschelt  and  Heider  (’92)  in 
regarding  the  sexual  organs  of  such  forms  as  the  Rhynchota 
(Aphidse,  Cicadidae)  and  Diptera  {Chironomtts,  Cecidomyid)  as 
derived  by  a  process  of  precocious  segregation  from  metameric 
gonads  like  those  of  the  Orthoptera.  These  exceptional  forms 
frequently  exhibit  peculiar  and  aberrant  features  (partheno¬ 
genesis,  paedogenesis)  like  the  Crustacea  which  have  a  similar 
precocious  segregation  of  germ-cells  {e.g.  Mohia^  according  to 
Grobben,  ’79). 

The  genital  ducts  of  the  insect  embryo  are  not  so  readily 
reduced  to  the  Annelid  type.  Many  authorities,  it  is  true, 
have  regarded  them  as  modified  nephridia  but  apart  from  their 
paired  mesodermal  origin  and  tubular  structure  there  was  very 
little  to  support  such  a  view.  ^  But  now  the  prevailing  view 
receives  fresh  support  from  the  fact  that  the  ducts  in  both 
sexes  arise  as  hollow  diverticula  of  the  coelom.  Though 
temporarily  obliterated  the  lumen  of  the  duct  is  very  probably 
a  persisting  remnant  of  the  coelomic  cavity.  This  is  certainly 

1  My  statement  in  a  former  paper  (’89)  that  the  genital  ducts  might  arise  from 
tracheal  involutions  is  erroneous.  What  I  saw  and  figured  (Fig.  80,  PI.  XIX) 
was  a  section  through  the  terminal  ampullae  of  the  deferent  ducts,  and  not  as  I 
supposed,  through  their  orifices. 


130 


WHEELER. 


[VOL.  VIII. 


the  case  with  the  cavities  of  the  terminal  ampullae  which  are 
never  obliterated. 

In  seeking  for  some  clue  to  the  true  nature  of  the  coelomic 
diverticula  one  naturally  turns  to  Peripatus.  Unfortunately 
the  two  accounts  of  the  development  of  the  nephridia  and  genital 
ducts  in  this  curious  Arthropod  —  the  one  by  v.  Kennel  (’85 
and  ’88),  the  other  by  Sedgwick  (’85  and  ’88)  — contradict  each 
other  in  many  particulars.  Both  authors,  however,  agree  in 
deriving  the  mesodermal  portion  of  the  nephridium  from  hollow 
diverticula  of  the  somites  (similar  to  those  seen  in  the  Orthop- 
tera,  in  that  they  extend  into  the  appendages ! ),  and  both  agree 
in  regarding  the  sexual  ducts  as  modified  nephridia.  But  Sedg¬ 
wick  derives  the  nephridium  from  the  portion  of  the  diverti¬ 
culum  located  in  the  appendage,  while  Kennel  derives  it  from 
the  inner  lower  angle  at  the  base  of  the  somite.  According  to 
Kennel  only  the  funnel-portion  arises  in  this  way,  the  long 
duct  being  formed  by  a  tubular  invagination  of  the  ectoderm. 
On  the  other  hand,  Sedgwick  derives  the  funnel  and  the  greater 
portion  of  the  duct  from  the  mesoderm  and  believes  that  only  a 
very  small  portion  of  the  duct  arises  by  invagination  from  the 
ectoderm.  These  differences  apply,  of  course,  to  the  sexual 
ducts  as  well.  According  to  v.  Kennel’s  account  not  only  their 
unpaired  terminal  portion  (opening  in  his  form  on  the  ante¬ 
penultimate  segment)  but  also  the  deferent  ducts  and  uteri 
are  ectodermal  ;  only  a  short  piece,  corresponding  to  the 
nephridial  funnel,  and  uniting  the  uteri  to  the  ovaries,  and  the 
deferent  ducts  to  the  testes,  has  a  mesodermal  origin.  Accord¬ 
ing  to  Sedgwick  the  dorsomedian  portion  of  the  coelom  persists 
in  the  segments  caudad  to  the  fifteenth  and  is  constricted  off 
from  the  remainder  of  the  somite.  The  dissepiments  are 
broken  down  between  the  adjacent  abstricted  portions  of  the 
somites,  so  that  a  hollow  tube  is  formed  on  either  side.  These 
tubes  receive  the  germ-cells  from  the  entoderm  and  form  the 
sexual  glands.^  In  the  segment  bearing  the  anal  papillae 

1  Sedgwick  claims  that  the  germ-cells  originate  in  the  entoderm  and  later  on 
migrate  into  the  coelomic  wall.  In  this  particular  I  prefer  to  adopt  v.  Kennel’s 
account,  according  to  which  the  germ-cells  have  a  mesodermal  origin,  since  it 
accords  better  with  the  facts  of  Annelid  development  and  with  my  own 
observations. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  131 

(which  in  all  probability  are  reduced  ambulatory  appendages) 
a  complete  separation  of  the  coelom  into  a  lateral  (diverticular) 
and  a  dorsomedian  (genital)  portion  does  not  take  place,  so 
that  the  two  cavities  remain  confluent.  The  portion  of  the 
coelomic  wall  surrounding  the  proximal  cavity  joins  the  sexual- 
gland  while  the  diverticular  (nephridial)  portion  acquires  an 
external  opening,  “which,  however,  is  much  nearer  the  middle 
line  than  in  the  case  of  the  anterior  somites,  and,  indeed,  may 
be  described  as  being  common  with  that  of  the  opposite  side. 
However  this  may  be,  the  two  openings  soon  become  definitely 
united  to  form  a  single  opening,  while  the  tubes  themselves 
persist  as  the  generative  ducts.  Whether  any  large  portion  of 
the  latter  are  ectodermal  in  origin,  that  is  to  say,  derived  from 
a  growth  of  the  lips  of  the  opening  at  its  first  appearance,  it  is 
impossible  to  say.” 

If  we  accept  Sedgwick’s  account  it  is  easy  to  reduce  the 
genital  ducts  of  insects  to  the  type  seen  in  Peripatus  and  con¬ 
sequently  to  Annelid  nephridia.  In  the  first  place,  everything 
goes  to  show  that  the  appendage  diverticula  of  the  coelom  are 
homologous  both  in  PeripaUts  and  Orthoptera.  In  both  cases 
the  oviducts  and  deferent  ducts  arise  from  these  diverticula  by 
partial  constriction.  Just  as  the  ducts  of  Peripatus  run  into 
the  cavities  of  the  anal  papillae,  so  the  sexual  ducts  of  Blatta 
and  Xiphidmm  run  into  the  rudimental  abdominal  appendages. 
In  both  cases  there  are  terminal  ampullae,  for  as  such  I  venture 
to  regard  the  slight  distal  widening  of  the  coelomic  diverticula 
in  Sedgwick’s  Figs.  42  and  44.  A  comparison  of  these 
figures  with  my  Figs.  56  and  59  will  show  the  close  resem¬ 
blance  between  insects  and  Peripatus  better  than  paragraphs 
of  description. 

As  the  exact  limits  of  the  ectodermal  portions  of  the  ducts 
of  PeripahLs  have  not  been  clearly  ascertained,  further  com¬ 
parison  with  the  Insects  cannot  at  present  be  undertaken.  In 
the  Insecta  only  the  vagina  and  ejaculatory  ducts  with  their 
respective  accessory  glands  arise  from  the  ectoderm.  These 
structures  are  median  and  unpaired  in  all  insects  except  the 
Ephemeridea,  one  of  the  oldest  and  most  primitive  groups.  In 
this  group,  as  Palmen  has  shown  (’84),  the  ducts  of  both  sexes 


132 


WHEELER. 


[VOL.  VIII. 


have  independent  openings.  The  oviducts  open  at  the  posterior 
edge  of  the  seventh,  the  deferent  ducts,  which  are  continued 
into  a  pair  of  penes,  at  the  posterior  edge  of  the  ninth  segment. 
There  is  no  ductus  ejaculatorius  proper  since,  according  to 
Palmen,  the  chitinous  cuticula  covering  the  surface  of  the  body 
does  not  extend  in  beyond  the  lips  of  the  orifices.^  In  the 
females  of  Heptage^iia  the  oviducts  open  at  the  bottom  of  an 
infolding  of  the  hypodermis  between  the  seventh  and  eighth 
segments.  This  infolding,  the  ovivalvula,  accommodates  the 
mature  eggs  till  the  time  for  oviposition,  and  may  be  regarded 
as  a  structure  on  the  way  to  becoming  a  vagina.  Morphologi¬ 
cally  it  is  simply  an  intersegmental  depression  differing  from 
those  which  separate  the  sternites  of  other  segments  only  in 
being  somewhat  exaggerated.  Palmen  observed  that  the  male 
genital  ostia  are  not  opened  till  the  last  nymphal  ecdysis. 

A  comparison  of  the  nymphal  Ephemerid  with  the  Ortho- 
pteran  embryo  is  very  instructive.  In  Xiphidmin  and  Blatta  the 
female  ampullm  lie  at  the  hind  end  of  the  seventh,  the  male  at 
the  hind  end  of  the  ninth  abdominal  segment.  Just  as  the 
deferent  ducts  of  Ephemerids  extend  into  the  penes  and  open  to 
the  exterior,  so  the  terminal  ampullae  originally  extend  into  a 
pair  of  appendages,  albeit  on  the  tenth  segment  and  not  opening 
to  the  exterior.  If  the  penes  of  Ephemerids  are  really  modified 
ambulatory  appendages  they  would  be  homologous  with  the 
styli  of  Orthoptera.  The  curious  persistence  of  these  append¬ 
ages  in  existing  Orthoptera  may  be  due  to  their  having  once 
functioned  as  penes,  long  after  the  other  abdominal  ambulatory 
appendages  had  disappeared.  It  would  be  necessary  to  suppose, 
if  this  view  were  adopted,  that  the  terminations  of  the  male 
ducts  had  moved  backwards.  But  this  whole  matter  is  very 

1  Palmen  claims  (’84,  p.  82)  to  have  found  no  chitinous  lining  in  the  terminal 
portion  of  the  ejaculatory  ducts  and  oviducts  of  Ephemerids  —  an  observation 
from  which  he  naturally  infers  that  these  ducts  are  mesodermal  throughout  their 
entire  length.  I  have  found,  however,  that  there  is  in  the  nymph  of  a  species  of 
Blasticrus  very  common  in  the  ponds  of  Worcester,  Mass.,  a  distinct  chitinous 
lining  to  the  ejaculatory  ducts  for  some  distance  inward  from  the  orifice  of  either 
penis.  My  attention  was  attracted  to  this  lining  during  the  ecdysis  of  the  insect, 
when  I  saw  the  membrane  withdrawn  from  the  ducts  along  with  the  cuticle 
covering  the  external  surfaces  of  the  penes  and  terminal  abdomuial  segments. 


No.  I.]  CONTRIBUTION  TO  INBECT  EMBRYOLOGY. 


133 


obscure,  for  why  should  the  ampullae  in  Xiphidium  move  from 
the  tenth  into  the  ninth  segment  ?  The  answer  to  this  enigma 
depends  on  further  comparative  embryological  research.  The 
long  persisting  closure  of  the  ostia  of  the  male  ducts  in  Ephe- 
merids  is  probably  an  embryonic  trait.  That  the  vagina  and 
ejaculatory  duct  of  higher  insects  may  have  arisen  from  a 
simple  intersegmental  depression  like  the  ovivalvula  receives 
some  support  from  the  fact  that  the  ectodermal  portions  of  the 
sexual  apparatus  make  their  appearance  so  late  ontogenetically. 
To  obtain  in  Xipliiditmi  a  condition  essentially  like  that  in 
Ephemerids  it  would  only  be  necessary  to  have  each  terminal 
ampulla  in  both  sexes  open  to  the  exterior. 

The  original  termination  of  the  sexual  ducts  in  modified 
ambulatory  appendages  —  which  is  so  clearly  seen  in  both 
sexes  in  embryonic  Orthoptera  —  is  very  probably  a  primitive 
feature.  In  the  Malacostraca  among  Crustacea  and  in  Dip- 
lopod  Myriopoda  the  sexual  ducts  terminate  on  more  or  less 
modified  ambulatory  limbs;  in  both  sexes  in  the  former  group, 
only  in  the  males  in  the  latter.  In  the  insect  embryo  the 
male  genital  appendages  are  larger  than  those  of  the  female; 
hence,  perhaps  the  larger  size  of  the  ampullae  filling  their 
cavities.  The  ampullae  are  probably  very  important  structures 
from  a  phylogenetic  standpoint.  They  may  perhaps  represent 
the  nephridial  Endblasen  of  Peripattis  and  Annelids.,  providing 
these  latter  structures  are  mesodermal.  In  Annelids  the  End¬ 
blasen  occasionally  function  as  temporary  receptacles  for  the 
sexual  products,  a  function  which  seems  to  have  been  retained 
in  the  male  insect,  where  they  become  the  vesiculae  seminales. 

Within  the  group  Eutracheata^  the  position  of  the  sexual 
openings  is  subject  to  great  variation.  Thus  in  Diplopods  and 
Pauropods  the  ducts  open  behind  the  second  pair  of  legs, 
usually  between  the  second  and  third  segments.  In  Chilo- 
poda,  on  the  other  hand,  they  open  on  the  penultimate  seg¬ 
ment.  In  the  Symphyla  the  unpaired  genital  orifice  is  situated 
on  the  fourth  segment,  which  probably  corresponds  to  the  first 
abdominal  segment  in  insects.  Even  within  the  division 
Apterygota  great  variation  is  observable.  In  the  Collembola  in 

^  U nder  this  heading  I  would  include  the  Myriopoda  and  Hexapoda, 


134 


WHEELER. 


[VOL.  VIII. 


both  sexes  the  orifice  lies  at  the  posterior  edge  of  the  fifth 
abdominal  segment.  In  the  Thysanura  the  female  opening 
usually  lies  on  the  eighth,  and  that  of  the  male  on  the  ninth 
abdominal  segment.  In  female  Orthoptera  and  Ephemeridea 
the  sexual  organs  open  behind  the  seventh,  in  the  male  behind 
the  ninth  abdominal  segment,  while  in  the  Plecoptera  and 
many  other  insects  the  female  orifice  is  said  to  lie  at  the  pos¬ 
terior  edge  of  the  eighth  segment.  Although  these  facts  of 
adult  anatomy  point  to  great  instability  in  the  segmental  ter¬ 
mination  of  the  sexual  ducts,  the  evidence  from  embryology  is 
more  conclusive.  The  female  Xiphidmm  embryo  has  at  first 
two  pairs  of  ducts,  and  in  the  male  the  single  pair  shift  their 
position  from  the  tenth  to  the  ninth  segment.  The  former 
fact  proves  conclusively  that  the  male  and  female  ducts  are 
not  homologous  but  homodynamous  structures,  and  the  latter 
that  ducts  may  shift  their  insertions  from  one  segment  to 
another  during  ontogeny.  The  inference  is,  that  sexual  ducts 
may  arise  in  any  nephridium-bearing  segment  from  the  pair 
of  nephridia  which  best  subserve  the  sexual  function  and  at 
the  same  time  interfere  least  with  the  development  and  func¬ 
tion  of  other  organs,  and  that  a  phylogenetic  shifting  of  the 
ducts  has  probably  taken  place  repeatedly.  The  position  of 
the  genital  ostia  on  a  particular  segment  cannot  therefore  be 
regarded  as  a  character  of  high  morphological  value,  at  least 
for  the  larger  groups.  . 

Two  conflicting  views  have  long  been  entertained  respecting 
the  morphological  significance  of  the  gonapophyses.  Under 
this  term,  introduced  by  Huxley  (’77),  we  may  include  the  ap¬ 
pendages  of  the  eighth  to  the  tenth  abdominal  segments  in  the 
female  and  such  of  their  homologues  as  persist  in  the  male. 
In  the  female,  these  appendages  go  to  form  the  ovipositor. 
According  to  Lacaze-Duthiers  (’49-53)  they  are  not  true  append¬ 
ages,  i.  e.  homodynamous  with  the  legs,  mouth-parts,  etc.,  but 
simply  modified  ventral  sclerites.  Haase  (’89),  too,  believes  that 
the  gonapophyses  are  not  true  appendages  but  Integument- 
bildungen  von  etwas  hoherer  Werthigkeit  als  die  Griffel,”  or 
styloid  processes  which  are  found  inserted  at  the  bases  of  the 
legs  in  some  Myriopods  and  Thysanura.  A  similar  view  ap- 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  135 

pears  to  be  held  by  Grass!  (’89).  All  these  authors  base  their 
conclusions  solely  on  comparative  anatomical  data. 

Other  observers,  including  Weismann  (’66),  Huxley  (’77), 
Uljanin,  Kowalevsky  (’73),  Kraepelin  (’72),  Dewitz  (’75)  and 
Cholodkovsky  (’9i^)  regard  the  gonapophyses  as  homodynamous 
with  the  true  ambulatory  appendages.  Most  of  these  authors 
adduce  support  for  their  views  from  the  origin  of  the  ovipositor 
during  the  larval  and  pupal  stages.  The  ovipositor  and  sting 
have  been  traced  in  Orthoptera  and  Hymenoptera  to  two  pairs 
of  imaginal  disks  —  one  situated  on  the  eighth,  the  other  on  the 
ninth  abdominal  segment.  On  the  latter  segment  the  pair  of 
disks  gives  rise  to  a  bifurcate  or  double  pair  of  appendages. 
(Dewitz,  Kraepelin,  etc.)  But  the  mere  fact  that  these  append¬ 
ages  arise  from  imaginal  disks  is  not  sufficient  evidence  of  their 
homodynamy  with  ambulatory  appendages,  since  the  wings  of 
the  Metabola  also  arise  from  imaginal  disks,  yet  cannot  belong 
to  the  same  category  as  the  ambulatory  appendages.  The 
imaginal  disks  of  the  gonapophyses  must  be  traced  into  the 
embryo  and  a  connection  clearly  established  between  them  and 
the  embryonic  appendages,  before  the  view  advocated  by 
Huxley,  Uljanin  and  others  can  be  said  to  rest  on  a  secure 
foundation.  Xiphidium  supplies  this  hitherto  missing  evidence. 
In  this  form  there  can  be  no  doubt  concerning  the  direct  con¬ 
tinuity  of  the  embryonic  appendages  with  the  gonapophyses. 
One  embryo  which  had  just  completed  katatrepsis  still  showed 
traces  of  all  the  abdominal  appendages.  The  pairs  on  the 
eighth,  ninth  and  tenth  segments  were  somewhat  enlarged. 
In  immediately  succeeding  stages  the  appendages  of  the  second 
to  sixth  segments  disappear  ;  the  pair  on  the  seventh  disappear 
somewhat  later.  Up  to  the  time  of  hatching  the  gonapophyses 
could  be  continuously  traced,  since  in  Xiphidium  there  is  no 
flexure  of  the  abdomen  as  in  other  forms  to  obscure  the  ventral 
view  of  the  terminal  segments.  From  the  time  of  hatching 
Dewitz  (’75)  has  traced  the  development  of  the  ovipositor  in 
another  Locustid  {Locusta  viridissimd)  so  that  now  we  have 
the  complete  history  of  the  organ. 

While  there  can  be  no  doubt  about  the  appendages  of  the 
eighth  and  ninth  segments,  which  go  to  form  the  two  outer 


136 


WHEELER. 


[VOL.  VIII. 


sheaths  of  the  ovipositor  or  sting,  the  development  of  the 
innermost  pair  of  blades  is  by  no  means  so  satisfactory.  But 
whether  this  pair  is  only  a  portion  of  the  ninth  pair  of  ap¬ 
pendages,  as  most  authors  claim,  or  represents  the  tenth  pair 
of  appendages,  as  I  maintain,  the  main  question  at  issue  is 
in  no  wise  affected  ;  for  it  still  remains  true  that  the  ovipositor 
consists  of  two  or  three  pairs  of  modified  ambulatory  limbs. 

In  the  male  Xiphidiuin  embryo  it  was  claimed  that  the  pair 
of  appendages  on  the  ninth  segment  persists  to  form  the  defini¬ 
tive  styli ;  those  of  the  eighth  and  tenth  segments  disappear¬ 
ing  very  early.  The  continuity  of  the  styli  with  the  embryonic 
appendages  was  quite  as  satisfactorily  observed  as  the  con¬ 
tinuity  of  the  ovipositor  blades.  Cholodkowsky  has  made  an 
exactly  similar  observation  on  Blatta  (91^).  The  styli  are, 
therefore,  the  homologues  of  the  second  pair  of  gonapophyses. 
Haase  must  therefore  have  gone  astray  in  seeking  to  homo- 
logize  the  styli  with  the  styloid  processes,  or  “  Griff  el,”  for 
the  styli  are  modified  ambulatory  appendages.  Moreover,  if 
my  interpretation  is  correct,  he  cannot  have  found,  as  he 
claims,  the  evanescent  rudiments  of  styli  in  young  female 
Blattids,  since  the  second  pair  of  ‘‘  anal  palps  ”  are  the  homo¬ 
logues  of  the  styli  (vide  Huxley,  ’77). 

VI I.  The  Subcesophageal  Body  in  Xiphidium  and  Blatta. 

This  structure,  of  which  I  have  elsewhere  (’92)  given  a  brief 
preliminary  account,  makes  its  appearance  in  the  Xiphidium 
embryo,  in  a  stage  a  little  earlier  than  F.  The  somites  in  the 
oral  and  thoracic  segments  are  then  established  as  closed  sacs. 
The  stomodseum  is  still  a  relatively  shallow  depression,  and 
the  entoderm-bands  starting  from  its  inner  end  have  made  but 
little  progress.  Sagittal  (Fig.  61)  and  frontal  sections  (Fig. 
62),  through  the  heads  of  embryos  in  Stage  F,  show  several 
interesting  details.  A  pair  of  somites  (coe)  lie  in  the  mandi¬ 
bular  segment,  and  previous  to  this  stage  there  was  also  a  pair 
of  small  somites  with  indistinct  cavities  in  the  tritocerebral 
segment  (/^).  The  planes  of  section  in  the  two  figures  are 
such  that  the  deutocerebral  somites  are  not  shown.  A  mass 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  137 


of  cells,  the  suboesophageal  body,  colored  pink  in  the  figure, 
extends  between  the  oesophagus  and  the  mandibular  somites. 
The  origin  of  this  mass  is  obscure.  It  may  arise  from  the 
ectoderm  of  the  oesophagus,  to  the  inner  end  of  which  it  is 
attached  (Fig.  61),  or  it  may  come  from  the  entoderm  I 

believe,  however,  that  it  arises  from  neither  of  these  sources, 
but  from  the  mesoderm,  which  in  a  preceding  stage  formed 
the  abortive  somites  of  the  tritocerebral  segment.  In  frontal 
section  (Fig.  62)  the  mass  of  cells  is  A-shaped,  with  the 
juncture  of  its  two  arms  attached  to  the  lower  surface  of 
the  oesophagus.  The  distal  ends  of  the  arms  are  applied  to 
the  anterior  walls  of  the  mandibular  somites.  The  separate 
cells  are  often  sharply  wedge-shaped  and  appear  to  be  sepa¬ 
rated  by  clear  spaces.  They  grow  somewhat,  lose  their  triangu¬ 
lar  outline  and  become  more  rounded.  At  the  same  time  they 
tend  to  fuse  in  curved  strings,  with  their  broad  edges  applied 
to  one  another.  This  condition  is  seen  in  Fig.  63,  which  is 
taken  from  a  section  through  the  organ  of  an  embryo  in  Stage 
G.  The  cytoplasm  is  now  very  granular,  and  has  a  distinctly 
yellow  tint  even  in  unstained  sections  ;  like  the  neuroblasts 
and  germ-cells  it  absorbs  picric  acid  with  avidity.  Vacuoles 
have  begun  to  make  their  appearance,  and  the  walls  between 
adjacent  cells  are  disappearing.  The  volume  of  the  nuclei 
remains  constant,  but  the  cytoplasm  enlarges  up  to  the  time 
of  hatching.  Fig.  64  is  a  part  of  a  section  through  the  sub¬ 
oesophageal  body  of  a  7  mm.  larva  of  Xiphidittm  fasciatum. 
The  condition  of  the  organ  is  essentially  the  same  as  at  the 
time  of  hatching.  The  increase  in  volume  of  the  cytoplasm  is 
clearly  shown.  Instead  of  being  granular,  as  in  the  younger 
stages,  the  protoplasm  is  now  so  filled  with  small  vacuoles  that 
it  is  reduced  to  a  coarse  reticulum. 

In  the  suboesophageal  body  of  a  larva  9  mm.  long,  signs 
of  degeneration  have  begun  to  appear.  The  small  vacu¬ 
oles  fuse  in  the  centres  of  the  cells,  leaving  only  the  cell- 
walls  as  ragged  envelopes.  The  nuclei  become  somewhat  poly¬ 
gonal  in  outline.  At  this  time  the  organ  is  found  attached  to 
the  anterior  ends  of  the  salivary  glands  and  to  the  large  trunks 
which  run  forward  into  the  head  from  the  first  thoracic  tracheae. 


138 


WHEELER, 


[VoL.  VIIL 


A  section  from  a  very  young  nymph  (lo  mm.  long),  is  shown 
in  Fig.  65.  The  cytoplasm  of  the  fused  cells  is  reduced 
to  a  ragged  mass  in  which  the  irregular  nuclei  are  suspended. 
Their  chromatin  is  aggregated  in  rounded  masses  —  a  sign  of 
advanced  degeneration.  In  this  stage  the  organ  is  much 
shrunken  in  size  so  that  one  is  led  to  conclude  that  part  of  it 
has  already  been  absorbed.  In  a  little  later  stage  the  last 
traces  of  the  organ  have  disappeared. 

A  suboesophageal  body  essentially  like  the  one  here  described 
occurs  also  in  Blatta.  It,  too,  has  the  characteristic  yellow 
tint.  In  his  study  of  the  development  of  Blatta  Cholodkowsky 
appears  to  have  seen  this  peculiar  structure,  though  he  regarded 
it  as  a  portion  of  the  fat-body.  At  page  52  (’91^),  he  says: 
“Die  Entodermlamelle  umwachst  den  Nahrungsdotter  dorsal- 
warts  und  von  alien  Seiten;  der  Vorder-  und  Hinterdarm 
liegen  nun  ausserhalb  des  Nahrungsdotters  und  werden  vom 
homogenen  Dotter  umspiilt,  in  welcJiem  {besonders  neben  dem 
CEsophagtts)  kleine  blasse  Zellen  liegen^  die  sick  in  Fettkorper  zu 
verwandeln  scJieinejiB  ^  The  organ  is  shown  in  Cholodkowsky’ s 
Fig.  68,  PI.  VI.  In  other  writers  on  insect  embryology  I  find 
no  mention  of  this  interesting  structure. 

In  the  Rhynchota,  to  judge  from  a  few  observations  on  the 
embryos  of  Zaitha  fluminea^  the  suboesophageal  body  occurs  in 
a  slightly  modified  form.  Here  it  consists  of  a  number  of  loose 
spherical  cells  lying  on  either  side  and  a  little  below  the 
oesophagus.  The  nuclei  are  large  and  spherical  and  the  com¬ 
pact  and  finely  granular  cytoplasm  has  a  distinct  yellow  cast. 
Though  these  cells  vary  in  size  (i  i— 15/a)  they  are  always  larger 
than  the  cells  of  the  surrounding  tissues  (6.3ia).  Beyond  this 
stage  I  could  not  trace  the  organ  in  Zaitha  on  account  of  lack 
of  material. 

The  suboesophageal  body  may  always  be  readily  distinguished 
from  the  fat-body  of  the  oral  and  more  posterior  segments  by 
the  peculiar  structure  and  arrangement  of  its  cells  and  by  its 
yellow  tint.  I  therefore  regard  it  as  an  organ  sui  genesis.  It 
belongs  to  the  category  of  embryonic  or  early  larval  organs. 


1  The  italics  are  mine. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  139 

and  this  alone  would  suffice  to  distinguish  it  from  the  fat-body 
which  persists  throughout  life. 

It  is  perhaps  premature  to  advance  any  hypothesis  as  to  the 
function  and  morphological  significance  of  the  suboesophageal 
body,  but  I  may  call  attention  to  its  possible  homology  with  an 
organ  in  the  Crustacea.  The  researches  of  Viallanes  and  St. 
Remy  go  to  show  that  the  tritocerebral  segment  of  insects  is 
homologous  with  the  second  antennary  segment  of  Crustacea. 
In  the  latter  group  of  Arthropods  this  segment  is  provided 
with  the  green-gland,  a  structure  which  develops  from  the 
mesoderm  and  is  generally  regarded  as  a  modified  nephridium. 
The  suboesophageal  body,  providing  it  arises  from  the  meso¬ 
derm  of  the  tritocerebral  segment,  may  be  all  that  remains 
of  this  same  pair  of  nephridia  in  the  cephalic  region  of 
insects. 


VIIL  Technique. 

Xiphidium  eggs,  like  those  of  other  Orthoptera  are  not 
easily  sectioned  in  the  younger  stages,  because  their  yolk 
bodies  are  rendered  so  brittle  by  the  hardening  fluids  and  are 
cemented  together  with  so  little  protoplasm  that  they  disin¬ 
tegrate  during  the  process  of  cutting.  After  the  appearance 
of  the  appendages  the  embryo  may  be  readily  dissected  away 
from  the  yolk  either  in  the  fresh  or  hardened  egg  and  mounted 
or  sectioned  by  itself.  In  the  study  of  the  envelopes,  where 
it  is  necessary  to  section  the  whole  egg,  the  following  method 
gives  fairly  good  results  :  — 

The  eggs  are  taken  from  the  galls  and  killed  by  being 
placed  for  about  a  minute  in  water  heated  to  80°  C.^  They 
are  then  transferred  for  preservation  to  70  per  cent  alcohol  in 
which  they  should  remain  for  several  weeks,  if  not  months,  in 
order  to  allow  the  yolk  to  harden  and  to  shrink  away  from  the 
chorion.  The  neglect  of  this  simple  precaution  has  led  many 
to  exaggerate  the  difficulty  of  studying  insect  eggs  or  to 
abandon  them  altogether.  After  remaining  in  the  alcohol 
for  some  time,  the  chorion  may  be  removed  by  tearing  it 


1  Alcoholic  picrosulphuric  acid  also  proved  to  be  an  excellent  killing  reagent. 


140 


WHEELER. 


[VOL.  VIII. 


open  at  the  broad  pole  and  gently  pushing  against  the  narrow- 
pole  of  the  yolk  with  one  needle,  while  holding  on  with 
the  other  to  the  chorion  at  the  same  pole.  In  the  earliest 
and  latest  stages  the  chitinous  blastodermic  membrane  comes 
off  with  the  chorion,  in  other  stages  it  adheres  firmly 
to  the  yolk  and  prevents  satisfactory  staining.  If  aqueous 
stains  like  Orth’s  lithium  carmine  or  Grenacher’s  alum  car¬ 
mine  are  used,  the  eggs  should  be  left  in  them  but  a  short 
time  and  carefully  watched  as  the  yolk-bodies  have  a  peculiar 
tendency  to  absorb  water  till  they  lose  the  polygonal  shapes 
they  acquired  by  mutual  pressure,  finally  swell  and  fall 
asunder.  This  is  especially  liable  to  occur  in  the  younger 
stages  when  the  blastodermic  membrane  is  removed.  I  have 
as  yet  found  no  other  insect  egg  with  yolk  capable  of  imbibing 
so  much  water.  In  Grenacher’s  borax  carmine  there  is  no 
swelling,  a  reason  which  has  induced  me  to  use  this  stain  in 
preference  to  the  aqueous  solutions  ;  though  the  two  stains 
mentioned  give  excellent  results  if  used  with  due  precautions. 
After  dehydrating  and  clearing  with  cedar  oil,  the  eggs  are 
kept  from  two  to  three  hours  in  melted  paraffine  (55°  C.). 
Older  embryos  in  which  most  of  the  yolk  has  been  metabolized 
need  not  remain  in  paraffine  more  than  an  hour. 

Embryos  isolated  from  the  yolk  in  the  anatreptic  stages,  as 
well  as  later  embryos  used  in  sectioning,  were  stained  in  Czo- 
kor’s  alum  cochineal.  The  bluish  color  of  this  stain  is  prefer¬ 
able  to  the  borax  carmine  in  serial  sections,  as  it  is  less 
wearisome  to  the  eye. 

In  the  study  of  the  entire  embryo  three  different  methods 
may  be  followed  with  advantage. 

Method  I,  —  The  isolated  embryo  is  stained  with  borax  car¬ 
mine,  all  excess  of  the  stain  is  removed  by  prolonged  immersion 
in  acid  alcohol,  and  the  preparation  mounted  in  clove  oil  or 
balsam.  In  such  preparations  many  of  the  details  of  internal 
structure,  such  as  the  arrangement  of  the  coelomic  sacs,  may 
be  very  clearly  distinguished.  This  method  was  very  exten¬ 
sively  used  by  Graber;  in  fact  it  seems  to  have  been  the  only 
method  which  he  employed  for  surface  study.  In  this  respect 
it  is  decidedly  inferior  to 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  141 

Method  II.  —  The  hardened  eggs  or  embryos,  freed  from 
their  envelopes,  are  transferred  from  seventy  per  cent  alcohol 
to  Delafield’s  or  Ehrlich’s  haematoxylin,  in  which  they  are  left 
not  longer  than  thirty  or  forty  seconds.  Then  they  are  suddenly 
returned  to  seventy  per  cent  alcohol,  and  a  drop  of  twenty  per 
cent.  HCl  is  allowed  to  fall  through  the  alcohol  onto  the  em¬ 
bryos,  which  almost  instantly  change  color.  As  soon  as  they 
pass  from  a  red  to  a  salmon  tint  the  fluid  must  be  hastily  re¬ 
moved  and  replaced  by  fresh  seventy  per  cent  alcohol,  to  which 
a  trace  of  ammonia  has  been  added.  The  nuclei  gradually  turn 
blue  and  throw  the  embryo  out  in  bold  contrast  to  the  pale 
yellow  yolk.  In  older  isolated  embryos,  the  stain  faintly  tinges 
the  surface  protoplasm,  accentuates  the  shadows,  and  leaves  all 
the  sharp  depressions  unstained.  When  embryos  thus  treated 
are  mounted  in  glycerin  or  balsam  and  examined  with  widely 
opened  diaphragm  and  Abbe  condenser  under  a  moderately 
low  power  (about  sixty  diameters),  the  surface  relief  is 
exquisitely  sharp  and  clear.  The  exact  delimitation  of  the 
appendages,  both  permanent  and  evanescent,  the  tracheal 
orifices,  oenocytic  invaginations,  segments  of  the  brain  and 
nerve-cord,  etc.,  may  be  traced  with  great  precision,  as  the 
figures  on  Plate  I  will  testify. 

The  method  here  given  with  several  modifications  of  my  own, 
was  taught  me  by  Dr.  Wm.  Patten,  who  has  used  it  with  great 
success  in  his  studies  of  Arthropod  development,  more  especially 
in  his  work  on  the  brain  and  eye  of  Acilius.  A  very  similar 
method  seems  to  have  been  used  by  other  investigators  {vide 
Foster  and  Balfour,  Elements  of  Embryology,  1883).  Un¬ 
fortunately,  surface  preparations  with  haematoxylin  are  not 
permanent,  probably  on  account  of  the  acid  used  to  extract 
the  stain.  The  color  gradually  fades,  often  disappearing 
completely  in  the  course  of  a  few  weeks.  I  therefore  prefer 
Czokor’s  alum  cochineal,  washing  in  water  instead  of  acidu¬ 
lated  alcohol.  These  preparations  are  nearly  or  quite  as  clear 
as  the  haematoxylin  preparations  and  keep  indefinitely. 

Method  III. — This  is  really  only  a  compromise  between 
Methods  I  and  II.  Embryos  in  the  katatreptic  stages  are 
allowed  to  remain  in  Czokor’s  alum  cochineal  till  the  stain  has 


142 


WHEELER. 


[VOL.  VIII. 


penetrated  as  far  as  but  not  into  the  yolk.  They  are  then 
washed  in  water,  dehydrated  and  mounted  in  balsam.  The 
sexual  ducts  together  with  their  ampullae  may  be  distinctly 
traced  on  the  yellow  background  of  the  yolk  and  structures 
which  lie  just  beneath  the  integument,  like  theoenocyte  clusters 
and  the  nerve-cord,  may  be  more  readily  studied  than  in 
specimens  prepared  by  Methods  I  and  II.  The  figures  on 
Plate  V  and  Fig.  10,  Plate  I  were  drawn  from  such  partially 
stained  embryos. 

The  methods  here  described  give  good  results,  not  only  with 
Xiphidium  and  Blatta^  but  also  with  all  the  other  insects  and 
crustaceans  which  I  have  examined. 

The  outlines  of  the  figures  in  the  plates  were  drawn  with 
an  Abbe  camera. 

Clark  University, 

Worcester,  Mass.,  May  loth,  1892. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  143 


IX.  BIBLIOGRAPHY. 

’88  Apathy,  S.  Analyse  der  ausseren  Kdrperform  der  Hirudineen.  Mit- 
theil.  a.  d.  zool.  Stat.  zu  Neapel.  8.  Bd.  1888. 

K’84  Ayers,  H.  On  the  Development  of  QEcanthus  niveus  and  its  Parasite 
Teleas.  Mem.  Bost.  Soc.  Nat.  Hist.  Vol.  3.  1884. 

><’80  Balfour,  F.  M.  Notes  on  the  Development  of  the  Araneina.  Quart. 
Jou?'7i.  Micr.  Sci.  Vol.  20.  1880. 

'88  Barrois,  J.  Recherches  sur  le  Ddveloppement  de  la  Comatule  (Coma- 
tula  mediterranea).  Recueil  zool.  Stiisse.  Tom.  4.  1888. 

’85  Bergh,  R.  S.  Die  Metamorphose  von  Aulastoma  gulo.  Arb.  zool. 
Inst.  Wurzburg.  7.  Bd.  1885. 

’87  Blochmann,  F.  Ueberdie  Richtungskdrper  bei  Insecteneiern.  Morph. 
Jahrb.  12.  Bd.  1887. 

’69  & ’70  Brauer,  F.  Verwandlung  der  Insecten  im  Sinne  der  Descendenz- 
Theorie.  Verhandl.  zool.  bot.  Gesell.  Wien.  I,  1869.  II,  1870. 
’85  Brauer,  F.  Systematisch  -  zoologische  Studien.  Sitz.-Ber.  Akad. 

Wiss.  Wien.  91.  Bd.  1885. 

’86  Brauer,  F.  Ansichten  iiber  die  palaozoischen  Insecten  und  deren 
Deutung.  Annal.  d.  k.  k.  fiaticrhist.  Hofinuseums.  Wien.  i.  Bd. 
1886. 

’89  Brongniart,  C.  Les  Blattes  de  I’dpoque  houilRre.  Cotfipt.  Re7td. 
Tom.  108.  1889. 

KlQl  Bruce,  A.  T.  Observations  on  the  Embryology  of  Insects  and  Arach¬ 
nids.  A  memorial  volume.  Baltimore,  1887. 

^91  Bumpus,  H.  C.  The  Embryology  of  the  American  Lobster.  Journ. 
of  Morph.  Vol.  5.  No.  2.  1891. 

’85  Carnoy,  J.  B.  La  Cytodierese  chez  les  Arthropodes.  La  Cellule. 
Tom.  I,  Ease.  2.  1885. 

’90  Carriere,  j.  Die  Entwicklung  der  Mauerbiene  (Chalicodoma  mu- 
raria,  Fabr.)  im  Ei.  Arch.  f.  77iikr.  A7iat.  35.  Bd.  1890. 

’91a  Cholodkowsky,  N.  Die  Embryonalentwicklung  von  Phyllodromia 
(Blatta)  germanica.  Me77i.  de  VAcad.  I77tp.  des  Sciences  de  St. 
Petersb.  7  ser.  Tom.  38.  No.  5.  1891. 

K’91t>  Cholodkowsky,  N.  Ueber  einige  Formen  des  Blastopors  bei  mero- 
blastischen  Eiern.  Zool.  Anzeig.  Jahrg.  14.  1891. 

’62  Claparede,  E.  Recherches  sur  I’Evolution  des  Araignees.  Naturk. 
Verhandel.  Deel  i,  Stuk  i.  Utrecht.  1862. 

^’91  Davenport,  C.  B.  Observations  on  Budding  in  Paludicella  and  some 
other  Bryozoa.  Btdl.  Mus.  Co77ip.  Zool.  Harvard  College.  Vol. 
XXII.  No.  I.  1891. 

’75  Dewitz,  H.  Ueber  Bau  und  Entwicklung  des  Stachels  und  der 
Legescheide  einiger  Hymenopteren  und  der  griinen  Heuschrecke. 
Zeitschr.  f,  wiss,  Zool,  25,  Bd.  1875. 


WHEELER, 


[VOL.  VIII. 


144 

’87  Eisig,  H.  Die  Capitelliden  des  Golfes  von  Neapel.  Fauna  und Flora 
des  Golfes  v.  Neapel.  Berlin.  1887. 

'77  Graber,  V.  .  Die  Insecten.  Naturkrafte.  22.  Bd.  Miinchen.  1877. 
’88a  Graber,  V.  Vergleichende  Studien  iiber  die  Keimhiillen  und  die 
Riickenbildung  der  Insecten.  Denkschr.  d.  niath.  naturw.  Classe 
d.  k.  Akad.  d.  Wzss.  Wien.  55.  Bd.  1888. 

’88tJ  Graber,  V.  Ueber  die  primare  Segmentirung  des  Keimstreifs  der 
Insecten.  Morph.  Jahrb.  14.  Bd.  1888. 

’89  Graber,  V.  Vergleichende  Studien  iiber  die  Embryologie  der  Insecten 
und  insbesondere  der  Musciden.  Denkschr.  d.  znath.  naturw.  Classe 
d.  k.  Akad.  d.  Wiss.  Wien.  56.  Bd.  1889. 

’90  Graber,  V.  Vergleichende  Studien  am  Keimstreif  der  Insecten. 
Denkschr.  d,  znath.  ziatzirw.  Classe  d.  k.  Akad.  d.  Wiss.  Wien. 
57.  Bd.  1890. 

’84  Grassi,  B.  Intorno  alio  sviluppo  delle  Api  nell’uovo.  AttidelVAcad. 

Gioeziia  di  Scienze  Nat.  izi  Catania.  3.  ser.  Vol.  18.  1884. 

’89  Grassi,  B.  Les  Ancetres  des  Myriapodes  et  des  Insectes.  Arch.  Ital. 
Biol.  Tom.  II.  1889. 

’79  Grobben,  C.  Die  Entwicklungsgeschichte  der  Moina  rectirostris, 
etc.  Arb.  zool.  Inst.  d.  Univ.  Wiezi.  2.  Bd.  1879. 

’89  Haase,  E.  Ueber  die  Zusammensetzung  des  Schabenkorpers.  Sitz- 
ungsb.  Gesell.  naturf.  Freuzide.  Berlin.  1889. 

^  ’89  Haase,  E.  Die  Abdominalanhange  der  Insecten,  mit  Beriicksichtigung 
der  Myriapoden.  Morph.  Jahrb.  15.  Bd.  1889. 

’85  Hallez,  P.  Orientation  de  I’Embryon  et  Formation  du  Cocon  chez  la 
Periplaneta  orientalis.  Coznpt.  Reztd.  Tom.  loi.  1885. 

’86  Hallez,  P.  Sur  la  Loi  de  I’Orientation  de  I’Embryon  chez  les  In¬ 
sectes.  Cozzzpt.  Rezzd.  Tom.  103.  1886. 

’77  Hatschek,  B.  Beitrage  zur  Entwicklungsgeschichte  der  Lepidopteren. 

Jena.  Zeitschr.  f.  Natzirw.  ii.  Bd.  1877. 

’84  Hatschek,  B.  Ueber  Entwicklung  von  Sipunculus  nudus.  Arb.  zool. 
Izzst.  Wiezz.  5.  Bd.  1884. 

K  ’89  Heider,  K.  Die  Embryonalentwicklung  von  Hydrophilus  piceus,  L. 
I.  Theil.  Jena.  1889. 

A  ’92  Herrick,  C.  L.  Additional  Notes  on  the  Teleost  Brain.  Azzat. 
Azzzeig.  7.  Jahrg.  June,  1892. 

<  ’86  Herrick,  F.  H.  Notes  on  the  Embryology  of  Alpheus  and  other 
Crustacea  and  on  the  Development  of  the  Compound  Eye.  Johzzs 
Hopkizzs  Uzziv.  Circ.  No.  6.  1886. 

K  ’90  Heymons,  R.  Ueber  die  hermaphroditische  Anlage  der  Sexualdriisen 
beim  Mannchen  von  Phyllodromia  (Blatta,  L.)  germanica.  Zool. 
Azzzeig.  13.  Jahrg.  1890. 

’91  Heymons,  R.  Die  Entwicklung  der  weiblichen  Geschlechtsorgane  von 
Phyllodromia  (Blatta)  germanica.  Zeitschr.  f.  wiss.  Zool.  53.  Bd. 
1891. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  145 


’89  His,  W.  Die  Neurobiasten  und  deren  Entstehung  im  embryonaien 
Mark.  Abhandl.  d.  k.  sacks.  Gesell.  Bd.  XV.  1889. 

’^'11  Huxley,  T.  H.  A  manual  of  the  Anatomy  of  the  Invertebrated 
Animals.  London,  1877. 

’88  Jordan,  K.  Anatomie  und  Biologie  der  Physopoda.  Zeitschr.  f. 
wiss.  Zool.  47.  Bd.  1888. 

’85  &  ’88  Kennel,  J.  v.  Entwicklungsgeschichte  von  Peripatus  Edwardsii, 
Blanch,  und  Peripatus  torquatus,  n.  sp.  i.  u.  2.  Theil.  Arb.  zool. 
zoot.  Inst.  Wurzburg.  7.  u.  8.  Bd.  1885  and  1888. 

’86  Kleinenberg,  N.  Die  Entstehung  des  Annelids  aus  der  Larve  von 
Lopadorhynchus.  Zeitschr.  f.  wiss.  Zool.  44  Bd.  1886. 

’81  Roller,  C.  Untersuchungen  iiber  die  Blatterbildung  im  Hiihnerei. 

Arch.f.  7nikr.  Anat.  20.  Bd.  1881. 

’85  Korotneff,  A.  Die  Embryologie  der  Gryllotalpa.  Zeitschr.  f.  wiss. 
Zool.  41.  Bd.  1885. 

K  ’92  Korschelt,  E.  und  Heider,  K.  Lehrbuch  der  vergleichenden  Ent¬ 
wicklungsgeschichte  der  wirbellosen  Thiere.  Spez.-Theil.  2.  Heft. 
1892. 

’72  Kowalevsky,  a.  (Note  on  Uljanin’s  researches  on  the  sting  of  the 
bee.)  Sitz.-Ber.  d.  zool.  Abth.  d.  3.  Versamml.  Naturf.  in  Kiew. 
Zeitschr.  f.  wiss.  Zool.  22.  Bd.  1872. 

V’86  Kowalevsky,  A.  Zur  embryonaien  Entwicklung  der  Musciden.  Bio¬ 
log.  Centralbl.  6.  Bd.  1886. 

’73  Kraepelin,  C.  Untersuchungen  iiber  den  Bau,  Mechanismus  und 
Entwicklungsgeschichte  des  Stachels  der  bienenartigen  Thiere. 
Zeitschr.  f.  wiss.  Zool.  23.  Bd.  1873. 

’49-’53  Lacaze-  Duthiers.  Recherches  sur  I’armure  gdnitale  femelles  des 
Insectes.  Ann.  d.  Sci.  Natur.  1849-1853. 

'  ’81  Lankester,  E.  Ray.  Appendages  and  Nervous  System  of  Apus  can- 
crif ormis.  Quart.  Journ.  Micr.  Sci.  Vol.  2 1 .  1881. 

’87  Lemoine,  V.  Recherches  sur  le  Ddveloppement  des  Podurelles.  Con- 
grls  de  la  Rochelle  (1882),  Ass.  Frang.  pour  VAvancement  des 
Sciences.  Paris.  1883.  (1887.) 

’92  Lenhossek,  M.  V.  Ursprung,  Verlauf  und  Endigung  der  sensibeln 
Nervenfasern  bei  Lumbricus.  Arch.  f.  inikr.  Anat.  39  Bd.  1892. 
’55  Leuckart,  R.  Ueber  die  Micropyle  und  den  feineren  Bau  der  Scha- 
lenhaut  bei  den  Insecteneiern.  Miill.  A rch.  f.  Anat.  u.  Phys.  1855. 
60  Leuckart,  R.  Bau  und  Entwicklungsgeschichte  der  Pentastomen. 
Leipzig  und  Heidelberg,  i860. 

'^’77  Mayer,  P.  Zur  Entwicklungsgeschichte  der  Decapoden. 

II.  Bd.  1877. 

’69  Melnikow,  N.  Beitrage  zur  Embryonalentwicklung  der  Insecten. 
Arch.f.  Naturgesch.  35.  Bd.  1869. 

f^’66  Metschnikoff,  E.  Embryologische  Studien  an  Insecten.  Zeitschr. 
f.  wiss.  Zool.  16.  Bd.  1866. 


[VOL.  VIII. 


146  WHEELER. 

’74  Metschnikoff,  E.  Embryologie  der  doppelfiissigen  Myriapoden 
(Chilognatha).  Zeitschr.  f.  wiss.  Zool.  24.  Bd.  1874. 

’86  Miall,  L.  C.,  and  Denny,  A.  The  Structure  and  Life-History  of  the 
Cockroach  (Periplaneta  orientalis).  London.  1886. 

’64  Muller,  F.  Fiir  Darwin.  1864. 
tA’83  Nusbaum,  J.  Vorlaufige  Mittheilung  iiber  die  Chorda  der  Arthro- 
poden.  Zool.  Anzeig.  6.  Jahrg.  1883. 

’84  Nusbaum,  J.  Rozwdj  przewodow  organdw  plciowych  u  owaddw. 
(Ueber  die  Entwicklungsgeschichte  der  Ausfiihrungsgange  der  Sexual- 
driisen  bei  den  Insecten.)  Lemberg.  1884. 

’87  Nusbaum,  J.  L’Embryologie  de  Mysis  Chameleo,  Thomps.  Arch, 
zool.  Exper.  Tom.  5.  1887. 

K’91a  Nusbaum,  J.  Beitrage  zur  Embryologie  der  Isopoden.  Biol.  Cen- 
tralbl.  II.  Bd.  1891. 

’91t>  Nusbaum,  J.  Studya  nad  Morfologia  Zwierzat.  1.  Przyczynek  do 
Embryologii  Maika  (Meloe  proscarabaeus,  Marsham).  1891. 

’91  Oka,  A.  Observations  on  Fresh-water  Polyzoa.  Journ.  Coll.  Sci. 

I7np.  Univ.  Japan.  Vol.  4.,  Pt.  i.  1891. 

’87  Oudemans,  j.  T.  Bijdrage  to  de  Kennis  der  Thysanura  en  Collembola. 
Academische  Proefschrift.  Amsterdam.  1887. 

K  ’83  Packard,  A.  S.  The  Embryological  Development  of  the  Locust. 

Third  Report  U.  S.  E7it.  Comm.  1883. 

A  ’84  Packard,  A.  S.  Article  on  Pseudoneuroptera.  The  Standard 
Natural  History.  Vol.  2.  1884. 

’84  Palm^n,  j.  a.  Ueber  paarige  Ausfiihrungsgange  der  Geschlechts- 
organe  bei  Insecten.  Helsingfors.  1884. 

K  ’90  Parker,  G.  H.  The  History  and  Development  of  the  Eye  in  the 
Lobster.  Bull.  Mus.  Co7np.  Zool.  Cambridge.  1890. 

M  ’84  Patten,  W.  The  Development  of  Phryganids  with  a  preliminary  note 
on  the  Development  of  Blatta  germanica.  Quart.  Journ.  Micr.  Sci. 
Vol.  24.  1884. 

H  ’88a  Patten,  W.  Studies  on  the  Eyes  of  Arthropods.  11.  Eyes  of  Aci- 
lius.  Jour,  of  Morph.  Vol.  2.  1888. 

/<’88t»  Patten,  W.  Segmental  Sense-Organs  of  Arthropods.  Journ.  of 
Morph.  Vol.  2.  1888. 

K  ’90  Patten,  W.  On  the  Origin  of  Vertebrates  from  Arachnids.  Quart. 
Journ.  Micr.  Sci.  Vol.  31.  1890. 

’86  Reichenbach,  H.  Studien  zur  Entwicklungsgeschichte  des  Fluss- 
krebses.  Abhandl.  Sencke7tb.  naturf.  Gesell.  14.  Bd.  1886. 

’86a  Ryder,  J.  A.  The  Origin  of  the  Amnion.  A7n.  Naturalist.  Vol.  20. 
1886. 

K ’86t>  Ryder,  J.  A.  The  Development  of  Anurida  maritima,  Gudrin.  Am. 
Naturalist.  Vol.  20.  1886. 

’90  St.  Remy,  G.  Contribution  a  I’etude  du  cerveau  chez  les  Arthropodes 
tracheates.  Arch.  zool.  ExJSr.  (2.)  Tom  5,  and  Supl.  (1887).  1890. 


No.  I.]  CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  147 

’87  SCHIMKEWITSCH,  W.  Etude  sur  le  Developpement  des  Araigndes. 
Arch,  de  Biol.  Tom.  6.  1887. 

K  ’85-88  Sedgwick,  A.  The  Development  of  the  Cape  Species  of  Peripatus. 
Part  I-VI.  Quart.  Journ.  Micr.  Sci.  Vol.  25-28.  1885-1888. 

K’91  Stiles,  C.  W.  Bau  und  Entwicklungsgeschichte  von  Pentastomum 
proboscideum  und  Pentastomum  subcylindricum.  Zeitschr.  f.  wiss. 
Zool.  52.  Bd.  1891. 

’82  Tichomiroff,  a.  The  Development  of  the  Silkworm  (Bombyx  mori) 
in  the  Egg.  (Russian.)  Works  Lab.  zool.  Mus.  Moskow.  Vol.  I. 
1882. 

’87a  ViALLANES,  H.  Sur  la  morphologie  du  cerveau  des  Insectes  et  des 
Crustaces.  Co7npt.  Rend.  Tom.  104.  1887. 

’87t>  ViALLANES,  H.  Etudes  Histologiques  et  Organologiques  sur  les 
centres  nerveux  et  les  organes  des  sens  des  animaux  articules. 
V.  Memoire.  Le  cerveau  du  Criquet  (Oedipoda  coerulescens  et 
Caloptenus  italicus),  etc.  Ami.  d.  Sci.  Natur.  Zool.  Tom.  4.  1887. 

’90  ViALLANES,  H.  Sur  quelques  Points  de  I’Histoire  du  Developpement 
embryonnaire  de  la  Mante  religieuse  (Mantis  religiosa).  Revue 
Biol,  du  Nord  de  la  France.  Tom.  2.  No.  12.  1890. 

’91  ViALLANES,  H.  Sur  quelques  Points  de  I’Histoire  du  Developpement 
embryonnaire  de  la  Mante  religieuse  (Mantis  religiosa).  Annal.  des 
Sci.  Natur.  Tom.  ii.  1891. 

’89  VOELTZKOW,  A.  Entwicklung  im  Eivon  Musca  vomitoria.  Arb.zooL 
zoot.  Inst.  Wurzburg.  9.  Bd.  1889. 

t\  ’90  Watase,  S.  On  the  Morphology  of  the  Compound  Eyes  of  Arthro¬ 
pods.  Stud.  Biol.  Lab.  Johns  Hopkins  Univ.  Vol.  4.  1890. 

’66  Weismann,  a.  Die  Metamorphose  der  Corethra  plmtiicornis.,  ein 
weiterer  Beitrag  zur  Entwicklungsgeschichte  der  Insecten.  Zeitschr. 
f.  wiss.  Zool.  16.  Bd.  1866. 

fC’OOa  Wheeler,  W.  M.  Ueber  driisenartige  Gebilde  im  ersten  Abdominal- 
segment  der  Hemipterenembryonen.  Zool.  Anz.  12.  Jahrg. 
1889. 

<  ’89t)  Wheeler,  W.  M.  The  Embryology  of  Blatta  germanica  and  Dory- 
phora  decemlineata.  Journ.  of  Morph.  Vol.  3.  1889. 

K  ’90a  Wheeler,  W.  M.  On  the  Appendages  of  the  First  Abdominal  Seg¬ 
ment  of  Embryo  Insects.  Trans.  Wis.  Acad.  Sci..,  Arts  and  Letters. 
Vol.  8.  1890. 

^  ’90t>  Wheeler,  W.  M.  Note  on  the  Oviposition  and  Embryonic  Devel¬ 
opment  of  Xiphidium  ensiferum.  Scud.  Insect  Life.  Vol.  2. 
Nos.  7  and  8.  1890. 

K"  ’90c  Wheeler,  W.  M.  Ueber  ein  eigenthiimliches  Organ  im  Locustiden- 
embryo.  Zool.  Anz.  13.  Jahrg.  1890. 

’91a  Wheeler,  W.  M.  The  Embryology  of  a  Common  Fly.  Psyche. 
June,  1891. 

’911)  Wheeler,  W,  M,  The  Germ-band  of  Insects.  Psyche.  July,  1891. 


148  WHEELER, 

K’9ic  Wheeler,  W.  M.  Neuroblasts  in  the  Arthropod  Embryo.  Journ. 
of  Morph.  Vol.  4.  1891. 

f  ’92  Wheeler,  W.  M.  Concerning  the  “  Blood-tissue  ”  of  the  Insecta. 
Psyche.  Feb.-April,  1892. 

K  ’78  Whitman,  C.  O.  The  Embryology  of  Clepsine.  Quar.  Journ.  Micr. 
Sci.  Vol.  18.  1878. 

A' ’87  Whitman,  C.  O.  A  Contribution  to  the  History  of  the  Germ-layers  in 
Clepsine.  Journ.  oj  M or Jh.  Vol.  i.  1887. 

’88  Will,  L.  Entwicklungsgeschichte  der  viviparen  Aphiden.  SpengeVs 
zool.  Jahrb.  Abth.  J.  Anat.  u.  Ontog.  3.  Bd.  1888. 
f\  ’81  Wilson,  E.  B.  The  Origin  and  Significance  of  the  Metamorphosis  of 
Actinotrocha.  Quart.  Journ.  Micr.  Sci.  Vol.  21.  1881. 

K  ’89  Wilson,  E.  B.  The  Embryology  of  the  Earth-worm.  Journ.  oj 
Morph.  Vol.  3.  1889. 

K  ’90  .  Wilson,  E.  B.  The  Origin  of  the  Mesoblast- Bands  in  Annelids. 
Journ.  oJ  Morph.  Vol.  4.  1890. 

’54  Zaddach,  G.  Untersuchungen  fiber  die  Entwicklung  und  den  Bau  der 
Gliederthiere,  I.  Die  Entwicklung  des  Phryganideneies.  Berlin. 

1854. 


I 

y 

t 


WHEELER. 


150 


EXPLANATION  OF  PLATE  1. 

LRiphidium  ensiferum,  Scud.) 

Fig.  I  {A).  Surface  view  of  embryo  during  gastrulation.  p.o.^  indusium  ; 
pcd,  procephalic  lobe;  <5/.,  blastopore;  a.,  anal  bifurcation  of  the  blastopore ; 
ams.,  amnioserosal  fold. 

Fig.  2  {B).  Surface  view  of  embryo  with  amnioserosal  fold  closed  over  trunk- 
region.  pcl.n.,  neuroblast-centres  on  the  procephalic  lobes.  0.,  anterior  widening 
of  the  blastopore.  Remaining  letters  as  in  Fig.  i. 

Fig.  3  {C).  Surface  view  of  embryo  with  the  amnioserosal  fold  encroaching 
on  the  indusial  thickening ;  po.am..,  amnioserosal  fold  of  the  indusium  ;  2;.,  pedicel 
temporarily  uniting  the  indusium  with  the  head ;  at.,  antenna ;  md.s.,  mandibular 
segment ;  first  maxillary  segment ;  mx.s.p  second  maxillary  segment ; 

p.s.,^-p.s.p  first  to  third  thoracic  segments  ;  a.s.,^  first  abdominal  segment.  Re¬ 
maining  letters  as  in  Fig.  i. 

Fig,  4  {B>).  Surface  view  of  embryo  just  after  the  separation  of  the  indusium 
from  the  head.  Letters  as  in  Figs.  3  and  i. 

Fig.  5  (E).  Indusium  spreading  over  the  yolk.  View  of  embryo  nearly 
completely  submerged  in  the  yolk,  on  its  way  to  the  dorsal  surface,  tc.s., 
tritocerebral  segment.  Other  letters  as  in  preceding  figures. 

Fig.  6  (B).  Surface  view  of  elongate  embryo  on  dorsal  surface  of  yolk,  lb., 
labrum  ;  md.,  mandible  ;  mx.^  first  maxilla  ;  mx.^  second  maxilla  ;  p.^  — p.,^  the 
three  thoracic  appendages  (legs);  coe.,  coelomic  sac  of  first  abdominal  segment 
showing  through  the  body  wall ;  pi.  (a/.^),  pleuropodium  (appendage  of  the  first 
abdominal  segment) ;  v.,  yolk  ;  envl.,  cellular  envelopes  torn  away  from  the  ventral 
face  of  the  embryo ;  cc.  (^?/.”),  cerci  (appendages  of  the  eleventh  abdominal 
segment). 

Fig.  7  {G).  Surface  view  of  shortened  embryo  on  dorsal  yolk,  pc.,^  second 
protocerebral  lobe  ;  pc.,'^  third  protocerebral  lobe  ;  dc.,  deutocerebrum  ;  tc.,  trito- 
cerebrum  ;  e.,  eye ;  x.,  metastigmatic,  or  oenocytic  invagination  ;  ap..,*  fourth  ab¬ 
dominal  appendage.  Remaining  letters  as  in  Fig.  6  (R). 

Fig.  8  (H).  Surface  view  of  embryo  turning  the  lower  pole  of  the  egg.  sr., 
inner  indusium  functioning  as  the  serosa ;  am.,  amnion  reflected  back  over  the 
yolk  and  continuous  with  the  membrane  sr. ;  at.,  antenna ;  pi.,  right  pleuro¬ 
podium  ;  igl.,  intraganglionic  thickening. 

Fig.  9  {/).  Surface  view  of  embryo  just  after  returning  to  the  ventral  face  of 
the  egg.  e.,  eye  ;  other  letters  as  in  Fig.  8  {H). 

Fig.  10  {K).  Surface  view  of  advanced  9  embryo  just  before  the  secretion  of 
the  larval  cuticle,  d.o.,  “  dorsal  organ  ”  ;  f.d.,  oviduct ;  ta.,  terminal  ampulla  of 
oviduct ;  op.^  {op.^),  op.^  {ap.'^),  first  and  second  pairs  of  gonapophyses  (append¬ 
ages  of  the  8th  and  9th  abdominal  segments),  cc.  (a/.”),  cercus. 


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WHEELER, 


EXPLANATION  OF  PLATE  11. 

(Figs.  II  and  12,  Stagmomantis  Carolina;  Figs.  13  and  14,  Gryllus  luctuosus ; 

Figs.  15-20,  Xiphidium  ensiferum.) 

Fig.  II.  Surface  view  of  gastrula  of  Stagmomantis.  X  150.  /.<?.,  rudiment 
of  indusium?;  bl.,  blastopore;  amnioserosal  fold  extending  just  over  the 

edge  of  the  oval  germ-band. 

Fig.  12.  Outline  of  egg  of  Stagmomantis  showing  the  position  and  relative 
size  of  the  germ-band  during  gastrulation. 

Fig.  13.  Surface  view  of  gastrula  of  Gryllus.  X  150.  ams.,  amnioserosal  fold 
extending  just  over  the  edge  of  the  germ-band  ;  bl..,  deeper  posterior  end  of  blasto¬ 
pore. 

Fig.  14.  Outline  of  egg  of  Gryllus  showing  the  relative  size  and  position  of 
the  germ-band  during  gastrulation. 

Fig.  15.  Surface  view  of  a  Xiphidium  embryo  during  the  closure  of  the 
amnioserosal  fold  over  the  mouth.  Eight  segments  in  the  trunk,  p.o.,  indusium  ; 
y.,  pale  area  between  the  indusium  and  the  head ;  pc.l.,  procephalic  lobe ;  ams., 
edge  of  amnioserosal  fold.  0.,  stomodaeum  ;  md.s.,  mandibular  segment ;  mx.s?-, 
first  maxillary  segment ;  n.g.,  neural  groove  ;  s.,  serosal  nuclei ;  a.s.,^  first  abdom¬ 
inal  segment. 

Fig.  16.  Surface  view  of  head  during  the  separation  of  the  indusium.  Stage 
of  Fig.  3  {C),  Plate  I  more  highly  magnified,  p.o.,  indusium ;  nn.,  shrunken  nuclei 
of  the  indusium  ;  s.,  serosal  nucleus  ;  z.,  pedicel  connecting  the  indusium  with  the 
head ;  pc.l.,  procephalic  lobe  ;  lb.,  labrum  ;  0.,  stomodaeum  ;  at.,  antenna ;  tc.s., 
tritocerebral  segment ;  md.s.,  mandibular  segment ;  mx.s.,^  first  maxillary  segment. 

Fig.  17.  Median  transverse  section  through  the  indusium  while  still  a  simple 
thickening  of  the  blastoderm  (serosa).  X  230.  s.,  serosa ;  p.o.,  portion  of  indu¬ 

sium  with  normal  nuclei ;  nn.,^  shrunken  nuclei ;  nn.,^  less  shrunken  nuclei ;  d., 
thickened  periphery  of  the  organ  ;  v.,  yolk. 

Fig.  18.  Median  transverse  section  of  the  indusium  while  the  amnioserosal 
fold  is  closing  over  the  disk.  X  ,230.  Letters  as  in  Fig.  17. 

Fig.  19.  Median  transverse  section  through  the  indusium  after  the  complete 
closure  of  the  amnioserosal  folds.  X  230.  a?n.,^  outer  indusial  layer ;  p.o.,  inner 

indusial  layer  ;  nn.,^  shrunken  nuclei. 

Fig.  20.  Median  transverse  section  through  the  indusium  just  after  it  has 
begun  to  spread.  X  230.  Letters  as  in  the  preceding  figures. 


PI.  II. 


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WHEELER, 


EXPLANATION  OF  PLATE  III. 

{Xiphidhim  ensiferum.) 

Fig.  21.  Median  longitudinal  section  through  the  head  of  an  embryo  over 
which  the  envelopes  have  just  closed.  X  230.  o.,  stomodaeum  ;  am.,  amnion  ; 

s,,  serosa ;  p.o.,  indusium  ;  z.,  pedicel  uniting  the  indusium  to  the  head  of  tho 
embryo  ;  ec.,  ectoderm  ;  ms.,  mesoderm  ;  vph.,  vitellophag  ;  v.,  yolk. 

Fig.  22.  Median  tranverse  section  of  the  indusium  when  it  has  reached  half 
way  round  the  egg.  X  230.  p.o.,  inner  indusial  layer  ;  am.,^  outer  indusial  layer  ; 
s.,  serosa. 

Fig.  23.  Three  normal  cells  from  the  indusium.  X  700. 

Fig.  24.  Three  cells  with  shrivelled  nuclei  from  the  indusium.  X  700. 

Fig.  25.  Transverse  section  through  basal  abdominal  region  of  an  embryo 
passing  to  the  dorsal  surface  of  the  yolk.  X  230.  nb.,  neuroblasts  ;  db.,  derma- 
toblasts  ;  am.,  amnion  ;  ms.,  mesoderm  ;  ec.,  ectoderm. 

Fig.  26.  Transverse  section  through  first  maxillary  segment  of  an  embryo 
passing  to  the  dorsal  surface  of  the  yolk.  X  230.  ng.,  neural  groove  ;  en.,  ento¬ 
derm.  Other  letters  as  in  Fig.  25. 

Fig.  27.  Transverse  section  through  the  second  maxillary  segment  of  an 
embryo  in  the  stage  of  Fig.  6,  (E)  Plate  I.  X  230.  mx.p  second  maxilla 
(trilobed)  ;  mnb.,  median-cord  neuroblast ;  ps.,  Punktsubstanz  ;  vph.,  vitellophag. 
Other  letters  as  in  Figs.  25  and  26. 

Fig.  28.  Transverse  section  through  the  mesothoracic  ganglion  of  an  embryo 
in  a  stage  somewhat  younger  than  G  (Fig.  7,  PI.  I).  X  230.  g.p  younger  offspring 
of  the  neuroblasts ;  g.,^  older  offspring  of  the  neuroblasts  (ganglion  cells) ;  ecd., 
ectoderm  ;  me.,  median  cord  (“  mittelstrang  ”).  Remaining  letters  as  in  preceding 
figures. 

Fig.  29.  Sagittal  section  through  nerve-cord  a  little  to  one  side  of  the  median 
line.  Embryo  in  Stage  G  (Fig.  7,  PL  I).  X  175.  md.g.,  mandibular  ganglion  ; 
mx.g.,^  first  maxillary  ganglion  ;  mx.g.p  second  maxillary  ganglion  ;  ig.,^  ig.,^  first 
and  second  interganglionic  depressions ;  p.g->^  first  thoracic  ganglion ;  p-g-? 
second  thoracic  ganglion  ;  mnb.,  median  cord  neuroblasts  ;  mg.,  their  progeny ; 
ini.,  inner  neurilemma. 

Fig.  30.  Frontal  section  through  the  base  of  the  abdomen  of  an  embryo 
somewhat  older  than  Fig.  6,  (F),  PI.  I.  X  175.  nb.,  neuroblasts;  mnb.,  median 
cord  neuroblasts  in  the  intersegmental  regions  ;  p.,^  metathoracic  leg ;  pL,  pleuro- 
podium  ;  ap.p-ap.,^  second  to  fifth  abdominal  appendages  grazed  by  the  knife. 

Fig.  31.  Transverse  section  through  the  mesothoracic  ganglion  of  an  embryo 
in  a  stage  between  Fig.  9  (_/)  and  10  {K)  PI.  I.  X  230.  hy.,  hypodermis  (product 
of  dermatoblasts)  ;  nb.,  neuroblasts  ;  g.^,  latest  progeny  of  the  neuroblasts  ;  g., 
ganglion-cells ;  ini.,  inner  neurilemma ;  enl.,  outer  neurilemma ;  ps.,  Punkt¬ 
substanz. 


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WHEELER, 


EXPLANATION  OF  PLATE  IV. 

{Xiphidium  ensiferum^ 

Fig.  32.  Transverse  section  through  the  head  of  an  embryo  in  the  stage  of 
Fig.  5  (E)  PL  I.  X  230.  /(5.,  labrum;  pc}  {o.g.),  pc.p  pc.p  first,  second  and  third 

protocerebal  lobes  ;  op.,  optic  plate  ;  en.  ms.,  mesentoderm. 

Fig.  33.  Next  following  section  to  that  represented  in  Fig.  32.  X  230.  n.b., 

neuroblast ;  am.,  amnion.  Remaining  letters  same  as  in  Fig.  32. 

Fig.  34.  Next  following  section  to  that  represented  in  Fig.  33.  X  230.  st., 
stomodaeum.  Remaining  figures  the  same  as  in  Figs.  32  and  33. 

Fig.  35.  Transverse  section  through  the  labrum,  in  a  stage  intermediate  be¬ 
tween  E  and  F  (Figs.  5  and  6,  PI.  I.)  X  230.  pc.,'^  third  protocerebal  lobe ;  lb., 
labrum  ;  ejt.  ms.,  mesentoderm. 

Fig.  36.  Transverse  section  through  labrum  and  brain  of  an  embryo  in  Stage 
F  (Fig.  6,  PI.  I.)  X  230.  Ib.y  labrum  ;  pc.p  pc.p pc., ^  ^xst,  second  and  third  proto¬ 
cerebral  lobes  ;  op.,  optic  plate  ;  nb.,  neuroblast ;  db.,  dermatoblasts  ;  coe.,  head- 
coelom  ;  ms.,  mesoderm  cells  ;  igl.,  intraganglionic  thickening  ;  st.,  stomodaeum  ; 
am.,  amnion. 

Fig.  37.  Transverse  section  through  praelabral  region  of  an  embryo  in  stage 
somewhat  later  than  F  (Fig.  6,  PI.  I).  X  145.  me.,  median  cord ;  pc}  (o.g.),  first 
protocerebal  lobe  (optic  ganglion)  ;  zo.,  orifice  of  involution  of  the  intraganglionic 
thickening.  Remaining  letters  as  in  Fig.  36. 

Fig.  38.  Transverse  section  through  optic  ganglion  and  optic  plate  of  an  em¬ 
bryo  in  Stage  G  (Fig.  7,  PI.  I).  X  145.  th.,  clear  thickening  in  the  optic  plate; 
e.,  eye  ;  on.,  optic  nerve  ;  pc}  {o.g.),  optic  ganglion  ;  am.,  amnion. 

Fig.  39.  Frontal  section  through  the  brain  of  an  embryo  somewhat  older  than 
I  (Fig.  9,  PI.  I).  X  175.  p.,  problematical  brain  segment;  pc}  {o.g.),  optic  gan¬ 
glion;  pc,.^  pc.,^  second  and  third  protocerebral  lobes;  dc.,  deutocerebrum ;  tc., 
tritocerebrum  ;  e.,  eye  ;  md.,  mandible  ;  md.g.,  mandibular  ganglion  ;  r.g.,  recurrent 
ganglion  ;  st.,  stomodaeum. 

Fig.  40.  Transverse  section  of  brain  through  the  supracesophageal  commis¬ 
sure  of  an  embryo  in  Stage  K  (Fig.  10,  PI.  I).  X  175,  nb.,  neuroblasts;  igl., 
interganglionic  thickening ;  th.,  clear  thickening  in  the  optic  plate ;  coe.,  head- 
coelom  ;  ps.,  Punktsubstanz  ;  on.,  optic  nerve.  Remaining  letters  as  in  Fig.  39. 

Fig.  41.  Transverse  section  from  same  series  as  that  represented  in  Fig.  40^ 
but  passing  through  the  frontal  ganglion.  X  175.  dc.,  deutocerebrum ;  f.g., 
frontal  ganglion ;  sb.,  suboesophageal  body.  Remaining  letters  as  in  Figs.  39 
and  40. 


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W.  M.  Wheeler  del.  Ittk.  Anst.  vYkrner  &  Mnter,  Amnkfurt 


158 


WHEELER. 


EXPLANATION  OF  PLATE  V. 

{Xiphidium  ensiferuni,  Figs.  42-46,  48-50.  X.  fasciatum.  Figs.  47  and  51.) 

Fig.  42.  Tip  of  abdomen  in  surface  view  from  a  $  embryo  which  has  just 
passed  the  lower  pole.  (Stage  J,  Fig.  9,  PI.  I.)  fifth  to  eighth  abdominal 

stigmata  ;  testis  ;  m.d.,  vas  deferens  ;  ta.m.,  terminal  ampulla ;  ap.P  appendages 
of  eighth  abdominal  segment ;  st.  {ap?),  stylets  (appendages  of  ninth  abd.  seg. ; 
ap.,^°  appendages  of  tenth  abdominal  segment,  in  the  cavities  of  which  the 
ampullae  lie  in  this  stage  ;  cc.  {ap.,'^^)  cerci.;  prd.,  proctodaeum  ;  an.,  anus. 

Fig.  43.  Tip  of  abdomen  in  surface  view  from  a  $  embryo  somewhat  older 
than  the  one  shown  in  Fig.  42.  Letters  same  as  in  Fig.  42. 

Fig.  44.  Tip  of  abdomen  in  surface  view  from  a  $  embryo  somewhat  older 
than  the  one  shown  in  Fig.  43.  v.,  yolk ;  ag.,  last  abdominal  ganglion.  Re¬ 

maining  letters  as  in  Fig.  42. 

Fig.  45.  Vasa  deferentia  {m.d.),  with  their  terminal  ampullae  {ta.tn.),  from  an 
embryo  just  before  the  development  of  the  larval  cuticle. 

Fig.  46.  Tip  of  abdomen  of  a  ^  embryo  ready  to  hatch.  Letters  same  as  in 
Figs.  42  -  44. 

Fig.  47.  Tip  of  abdomen  opened  and  seen  from  within  from  a  ^  larva  i  cm. 
long,  m.o.,  sexual  orifice,  c.n.,  connectives  ;  remaining  letters  as  in  Figs.  42-44. 

Fig.  48.  Tip  of  abdomen  of  9  embryo  in  a  stage  corresponding  to  that 
represented  in  Fig.  42.  ov.,  ovary  ;  f.d.,  oviduct ;  ta.f.,  terminal  ampulla ;  m.d., 
vas  deferens  and  terminal  ampulla  of  the  male  type,  still  persisting;  sixth 

to  eighth  abdominal  stigmata;  ap.^  persisting  appendage  of  the  seventh  abdominal 
segment ;  op.^  {(ip‘^)i  op.^  i^p-^),  op.^  {ap.'^°),  three  pairs  of  abdominal  appendages 
which  become  the  gonapophyses  (ovipositor)  ;  an.,  anus  ;  cc.  (ap.^^),  cerci ;  z/.,  yolk. 

Fig.  49.  Tip  of  abdomen  of  9  embryo  seen  in  full  surface  view  in  Fig.  10.  (X), 
PI.  I.  mnb.,  location  of  median  cord  neuroblast ;  cm.,  posterior  commissure  ;  cn., 
connective  ;  ag.,  last  abdominal  ganglion.  Other  letters  as  in  Fig.  48. 

Fig.  50.  Tip  of  abdomen  of  9  embryo  ready  to  hatch.  Letters  same  as  in 
Figs.  48  and  49. 

Fig.  51.  Tip  of  abdomen  of  9  larva  i  cm.  long,  opened  and  seen  from  within; 
all  the  parts  being  dissected  away  except  the  reproductive  organs,  vg.,  vagina ; 
op.'^  {ap.^),  op.^  {ap‘^),  op.^  three  pairs  of  gonapophyses.  Other  letters  as 

in  Fig.  48. 


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CONTRIBUTION  TO  INSECT  EMBRYOLOGY.  159 


EXPLANATION  OF  PLATE  VI. 

{Xiphidium  ensifertwi.  Figs.  52-63  ;  X.  fas  datum,  Figs.  64  and  65). 

Fig.  52.  Frontal  section  through  first  to  fourth  abdominal  segments,  show¬ 
ing  segmental  arrangement  of  the  gonads.  Embryo  in  Stage  F  (Fig.  6,  PL  I). 
X  230.  ml.,  longitudinal  ventral  muscle  ;  gd.^gd.^gd.,^  gonads  of  first,  second  and 
third  abdominal  segments  ;  coe.,  coelomic  cavity ;  ecd.,  ectoderm  ;  ep.,  epithelium. 

Fig.  53.  Transverse  section  through  the  third  abdominal  segment  of  an 
embryo  in  Stage  F,  (Fig.  6,  PI.  I).  X  230.  nc.,  nerve  cord  ;  am.,  amnion,  sms., 
somatic  wall  of  somite  ;  spms.,  splanchnic  wall  of  somite  ;  v.,  yolk  ;  en.,  ento¬ 
derm.  Remaining  letters  as  in  Fig.  52. 

Fig.  54.  Frontal  section  through  the  fourth  abdominal  segment  of  an  embryo 
in  Stage  F  (Fig.  6,  PI.  I).  X  230.  The  diverticula  point  towards  the  head,  v., 
yolk  ;  gd.,^  gonad  of  fourth  abdominal  segment ;  coe.,  coelomic  cavity  ;  ep.,  epithe¬ 
lium. 

Fig.  55.  Section  through  a  somite  from  the  third  thoracic  segment  showing 
a  single  enlarged  germ-cell  protruding  into  the  coelomic  cavity.  X  230. 

Fig.  56.  Sagittal  section  through  the  end  of  the  abdomen  of  an  embryo  in 
Stage  G  (Fig.  7,  PI.  I),  tb.,  neuroteloblast  ? ;  nb.,  neuroblasts;  coe.'^-coe.,^°  coel¬ 
omic  cavities  of  the  seventh  to  tenth  abdominal  segments  ;  7n.d.,  diverticulum  of  the 
tenth  abdominal  somite  which  becomes  the  vas  deferens  and  its  terminal  ampulla ; 
gd.,^°  gonad  in  tenth  abdominal  segment  (abnormal  and  atavistic);  n.c.,  nerve  cord 
in  unflexed  portion  of  abdomen. 

Fig.  57.  Transverse  section  through  ninth  abdominal  segment  of  embryo 
represented  in  Fig.  42,  PI.  V,  cutting  the  tenth  pair  of  appendages.  X  175.  msc., 
muscular  tissue  ;  prd.,  proctodaeum  ;  n.c.,  nerve  cord  ;  h.,  heart ;  ecd.,  ectoderm  ; 
m.d.,  vas  deferens  ;  ta.m.,  terminal  ampulla ;  ap.,^°  appendage  of  the  tenth  ab¬ 
dominal  segment. 

Fig.  58.  Transverse  section  of  the  abdomen, of  an  embryo  in  the  stage  repre¬ 
sented  in  Fig.  48, .PL  V.  The  greater  portion  of  the  section  passes  through  the 
ninth,  its  anterior  portion  through  the  tenth  abdominal  segment.  X  175.  coe.,^° 

coelom  of  tenth  abdominal  segment ;  bl.,  blood  corpuscle  ;  ta.m.,  terminal  ampulla 
of  vas  deferens  ;  m.d.,  vas  deferens  disintegrating  ;  op,^  {ap.^°),  third  pair  of  gona- 
pophyses  ;  n.c.,  nerve  cord  ;  prd.,  proctodaeum  ;  ec.,  ectoderm  ;  k.,  heart. 

Fig.  59.  Transverse  section  through  the  seventh  abdominal  segment,  taken 
from  the  same  embryo  as  the  section  in  Fig.  58.  X  175.  v.,  yolk;  en.,  ento¬ 

derm  ;  bl.,  blood-corpuscle  dividing ;  h.,  heart ;  coe.P  coelom  of  the  seventh 
abdominal  segment ;  f.d.,  oviduct ;  ta.,  terminal  ampulla ;  oe.,  oenocytes ;  ec., 
ectoderm  ;  nc.,  nerve-cord  ;  ap.p  appendage  of  seventh  abdominal  segment. 

Fig.  60.  Transverse  section  through  the  seventh  abdominal  segment  of  an 
embryo  somewhat  older  than  that  .in  Fig.  48,  PL  V.  X  175.  ov.,  ovary;  me., 
median  cord  ;  ad.,  fat-body ;  spms.,  splanchnic  mesoderm.  Other  letters  as  in 
Fig.  59- 

Fig.  61.  Sagittal  section  through  the  head  of  an  embryo  in  Stage  F  (Fig.  6, 
PL  I).  X  175.  frontal  ganglion  ;  first  recurrent  ganglion  ;  second 

recurrent  ganglion  ;  v,,  yolk  ;  pc.,^  third  protocerebral  lobe  ;  ms.,  mesoderm  ;  lb., 


i6o 


WHEELER. 


labrum ;  j/.,  stomodseum ;  tc.^  tritocerebrum ;  md.g.,  mandibular  ganglion ;  en.^ 
entoderm  ;  s.b.y  subcesophageal  body  ;  coe-d"  coelom  of  mandibular  segment. 

Fig.  62.  Frontal  section  through  the  head  of  an  embryo  in  Stage  F  (Fig.  6, 
PI.  I).  X  175.  pc.'‘-  {o.g),  optic  ganglion  ;  md.^  mandible.  Remaining  letters  as 
in  Fig.  61. 

Fig.  63.  Transverse  section  through  the  subcesophageal  body  from  an  em¬ 
bryo  in  Stage  G  (Fig.  7,  PI.  I.).  X  500. 

Fig.  64.  Section  through  subcesophageal  body  of  larva  7  mm.  long.  X  500. 

Fig.  65.  Section  through  the  subcesophageal  body  of  a  nymph  i  cm.  long. 
X  500. 


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